meson: undefine NDEBUG in the tests

[evelikov]

When using -D b_ndebug=true or inheriting CFLAGS="-DNDEBUG" from the environment, the asserts will be no-op thus the checks in the LD_PRELOADED libraries will be omitted.

Explicitly undefine the macro, so we don't accidentally shoot ourselves in the foot.

---

As an aside: our tests are fairly inconsistent, namely

• explicit return/fail vs assert_return(.... EXIT_FAILURE)
• close/free in error paths vs always leak
• return 0 vs return EXIT_SUCCESS vs exit(EXIT_SUCCESS)

Any objections if we get them on the same page, any preferences on the individual sub-topics?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

[stoeckmann]

• return 0 vs return EXIT_SUCCESS vs exit(EXIT_SUCCESS)

Definitely prefer return because calling exit would skip a stack canary check. This check should be called as often as possible.

[lucasdemarchi]

Applied, thanks