cmake: allow building tests in unity mode

Makes building tests noticeably faster.

Apply changes/fixes/workarounds to make Unity work:
- rename test variables to avoid collisions or shadowing each other when
  combined into single units.
- add workaround to avoid applying `lib/memdebug.h` overrides to system
  headers declaring/defining `getaddrinfo()`/`freeaddrinfo()` for
  `tests/server/resolve.c`. This replaces a previous workaround that
  worked for that specific source.
- rename test macro `CTRL` clashing with Cygwin `sys/ioctl.h`.
- add include guard to `test.h`.

Also:
- exclude `tests/http/clients` which are all single-source. (like
  `docs/examples`.)

Build time improvements for tests:
- AppVeyor CI:
  - MSVC 2008, 2010: 1 minute faster (4m8s -> 2m56s, 3m19s -> 2m24s)
  - MSVC 2022 arm64: 3.5 minutes faster (10m18s -> 6m48s)
  before: https://ci.appveyor.com/project/curlorg/curl/builds/50522785
  after: https://ci.appveyor.com/project/curlorg/curl/builds/50522942
- GHA:
  - Cygwin: 1.5 minutes faster (3m13s -> 1m43s)
    before: https://github.com/curl/curl/actions/runs/10681535327/job/29605384398
    after: https://github.com/curl/curl/actions/runs/10680818726/job/29603130637
  - Windows:
    before: https://github.com/curl/curl/actions/runs/10680818713
    after: https://github.com/curl/curl/actions/runs/10683850187
    - MSYS2, mingw-w64: 1 minute faster
    - MSVC: 30 seconds faster (3m17s -> 2m48s)
  - macOS: double speed (39s -> 18s)
    before: https://github.com/curl/curl/actions/runs/10680818753/job/29603133447
    after: https://github.com/curl/curl/actions/runs/10683850174/job/29612914515
  - Linux: almost double speed (30/31s -> 18s)
    before: https://github.com/curl/curl/actions/runs/10681535311/job/29605387156
    after: https://github.com/curl/curl/actions/runs/10680818721/job/29603133976
  - non-native: no obvious effect.
    before: https://github.com/curl/curl/actions/runs/10680818722
    after: https://github.com/curl/curl/actions/runs/10683850187
  - Old Linux: Unity mode not supported by old CMake, no effect.

Closes #14765

tests/unit/unit1652.c

@@ -37,7 +37,7 @@
 static struct Curl_easy *data;
 
 static char input[4096];
-static char result[4096];
+static char output[4096];
 
 int debugf_cb(CURL *handle, curl_infotype type, char *buf, size_t size,
               void *userptr);
@@ -55,8 +55,8 @@ debugf_cb(CURL *handle, curl_infotype type, char *buf, size_t size,
   (void)type;
   (void)userptr;
 
-  memset(result, '\0', sizeof(result));
-  memcpy(result, buf, size);
+  memset(output, '\0', sizeof(output));
+  memcpy(output, buf, size);
   return 0;
 }
 
@@ -97,47 +97,46 @@ UNITTEST_START
 /* Injecting a simple short string via a format */
 msnprintf(input, sizeof(input), "Simple Test");
 Curl_infof(data, "%s", input);
-fail_unless(verify(result, input) == 0, "Simple string test");
+fail_unless(verify(output, input) == 0, "Simple string test");
 
 /* Injecting a few different variables with a format */
 Curl_infof(data, "%s %u testing %lu", input, 42, 43L);
-fail_unless(verify(result, "Simple Test 42 testing 43\n") == 0,
+fail_unless(verify(output, "Simple Test 42 testing 43\n") == 0,
             "Format string");
 
 /* Variations of empty strings */
 Curl_infof(data, "");
-fail_unless(strlen(result) == 1, "Empty string");
+fail_unless(strlen(output) == 1, "Empty string");
 Curl_infof(data, "%s", (char *)NULL);
-fail_unless(verify(result, "(nil)") == 0, "Passing NULL as string");
+fail_unless(verify(output, "(nil)") == 0, "Passing NULL as string");
 
 /* A string just long enough to not be truncated */
 memset(input, '\0', sizeof(input));
 memset(input, 'A', 2047);
 Curl_infof(data, "%s", input);
-fail_unless(strlen(result) == 2048, "No truncation of infof input");
-fail_unless(verify(result, input) == 0, "No truncation of infof input");
-fail_unless(result[sizeof(result) - 1] == '\0',
+fail_unless(strlen(output) == 2048, "No truncation of infof input");
+fail_unless(verify(output, input) == 0, "No truncation of infof input");
+fail_unless(output[sizeof(output) - 1] == '\0',
             "No truncation of infof input");
 
 /* Just over the limit without newline for truncation via '...' */
 memset(input + 2047, 'A', 4);
 Curl_infof(data, "%s", input);
-fail_unless(strlen(result) == 2051, "Truncation of infof input 1");
-fail_unless(result[sizeof(result) - 1] == '\0', "Truncation of infof input 1");
+fail_unless(strlen(output) == 2051, "Truncation of infof input 1");
+fail_unless(output[sizeof(output) - 1] == '\0', "Truncation of infof input 1");
 
 /* Just over the limit with newline for truncation via '...' */
 memset(input + 2047, 'A', 4);
 memset(input + 2047 + 4, '\n', 1);
 Curl_infof(data, "%s", input);
-fail_unless(strlen(result) == 2051, "Truncation of infof input 2");
-fail_unless(result[sizeof(result) - 1] == '\0', "Truncation of infof input 2");
+fail_unless(strlen(output) == 2051, "Truncation of infof input 2");
+fail_unless(output[sizeof(output) - 1] == '\0', "Truncation of infof input 2");
 
 /* Way over the limit for truncation via '...' */
 memset(input, '\0', sizeof(input));
 memset(input, 'A', sizeof(input) - 1);
 Curl_infof(data, "%s", input);
-fail_unless(strlen(result) == 2051, "Truncation of infof input 3");
-fail_unless(result[sizeof(result) - 1] == '\0', "Truncation of infof input 3");
-
+fail_unless(strlen(output) == 2051, "Truncation of infof input 3");
+fail_unless(output[sizeof(output) - 1] == '\0', "Truncation of infof input 3");
 
 UNITTEST_STOP