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

lib/escape.c

@@ -223,8 +223,6 @@ void Curl_hexencode(const unsigned char *src, size_t len, /* input length */
   DEBUGASSERT(src && len && (olen >= 3));
   if(src && len && (olen >= 3)) {
     while(len-- && (olen >= 3)) {
-      /* clang-tidy warns on this line without this comment: */
-      /* NOLINTNEXTLINE(clang-analyzer-core.UndefinedBinaryOperatorResult) */
       *out++ = (unsigned char)hex[(*src & 0xF0) >> 4];
       *out++ = (unsigned char)hex[*src & 0x0F];
       ++src;