ztest: undefine _POSIX_C_SOURCE before defining it

Undefine _POSIX_C_SOURCE before defining it to avoid double
definition

The previous version, changed in this PR
https://github.com/zephyrproject-rtos/zephyr/pull/70363

, did the undef.
Make sure to do the same for all files in the current dir.

Signed-off-by: Dawid Niedzwiecki <dawidn@google.com>