tests: make first.c the same for both lib tests and unit tests

tests: make first.c the same for both lib tests and unit tests

The only difference used to be global variable used in unittest tests.
After cb7ed5a removed individual flag overrides for the unittests, first.c
was no longer recompiled for unit tests to include the flag, so whether it
worked or gave a link error depended on whether it was compiled in
libtest or unittest first. This way also speeds up the build by
eliminating 40 identical compile invocations.

Fixes #10749