cmake: replace the way clang-tidy verifies tests, fix issues found

[vszakats]

Replace existing mk-unity.pl --embed workaround with running
clang-tidy manually on individual test source instead. This aligns
with how clang-tidy works and removes mk-unity.pl from the solution.

Also:

• mqttd: fix potentially uninitialized buffer by zero filling it.

  tests/server/mqttd.c:484:41: error: The left operand of '<<' is a garbage value
    [clang-analyzer-core.UndefinedBinaryOperatorResult,-warnings-as-errors]
    484 |       payload_len = (size_t)(buffer[10] << 8) | buffer[11];
        |                                         ^
  [...]
  tests/server/mqttd.c:606:45: error: The left operand of '<<' is a garbage value
    [clang-analyzer-core.UndefinedBinaryOperatorResult,-warnings-as-errors]
    606 |       topiclen = (size_t)(buffer[1 + bytes] << 8) | buffer[2 + bytes];
        |                                             ^
  

• sockfilt: fix potential out-of-bound pointer:

  tests/server/sockfilt.c:1128:33: error: The 2nd argument to 'send' is a buffer
     with size 17010 but should be a buffer with size equal to or greater than
     the value of the 3rd argument (which is 18446744073709551615)
     [clang-analyzer-unix.StdCLibraryFunctions,-warnings-as-errors]
   1128 |         ssize_t bytes_written = swrite(sockfd, buffer, buffer_len);
        |                                 ^
  

• clang-tidy: suppress bogus bzero() warnings that happens
  inside the notorious FD_ZERO() macros, on macOS.

Ref: #17680 (comment)

---

The manual clang-tidy solution had two ugly workarounds → [REPLACED WITH CODE]:

• needed to pass UNITTESTS macro manually for units, tunits for
  clang-tidy, because I couldn't extract this from CMake, where it's
  inheriting it from the libcurlu, libcurltool libraries. Perhaps
  there is a way to automate this more.
• needed to manually tweak the source filename for the generated
  lib1521.c. Similar trick is already used for src in ./configure.