cmake: fix clang-tidy builds to verify tests, fix fallouts

- cmake: disable test bundles for clang-tidy builds.
  clang-tidy ignores #included .c sources, and incompatible with unity
  and bundles. It caused clang-tidy ignoring all test sources. It also
  means this is the first time tests sources are checked with
  clang-tidy. (autotools doesn't run it on tests.)

- cmake: update description for `CURL_TEST_BUNDLES` option.

- fix tests using special `CURLE_*` enums that were missing from
  `curl/curl.h`. Add them as reserved codes.

- fix about ~50 other issues detected by clang-tidy: unchecked results,
  NULL derefs, memory leaks, casts to enums, unused assigments,
  uninitialized `errno` uses, unchecked `open`, indent, and more.

- drop unnecessary casts (lib1533, lib3207).

- suppress a few impossible cases with detailed `NOLINT`s.

- lib/escape.c: drop `NOLINT` no longer necessary.
  Follow-up to 72abf7c13a #13862 (possibly)

- extend two existing `NOLINT` comments with details.

Follow-up to fabfa8e402 #15825

Closes #16756

tests/unit/unit1661.c

@@ -100,8 +100,10 @@ UNITTEST_START
   buffer = (const char *)Curl_bufref_ptr(&bufref);
   fail_unless(buffer, "Allocated pointer is NULL");
   fail_unless(bufref.len == 3, "Wrong data size stored");
-  fail_unless(!buffer[3], "Duplicated data should have been truncated");
-  fail_unless(!strcmp(buffer, "166"), "Bad duplicated data");
+  if(buffer) {
+    fail_unless(!buffer[3], "Duplicated data should have been truncated");
+    fail_unless(!strcmp(buffer, "166"), "Bad duplicated data");
+  }
 
   /**
    * testing Curl_bufref_free