[Tests] do not use Frama_C_foo directly

Instead, include the proper libc headers, with the specifications.
Otherwise, in the next commits(s), we won't have preconditions to evaluate, and the tests
will be come unsound

Changes:

- share/libc/stdlib.c: previous version of calloc in stdlib. used Frama_C_memset, which is idiotic since
  - for non-free we now *have* a builtin for calloc
  - stdlib.c is meant for people with non-free builtins

- same thing for tests/libs/string_c.c: the builtin was used

- nonfree/imprecise.c:
  sizeof(v3), where v3 is an abstract struct, is evaluated slightly less precisely now.
  We don't care as long as it does not crash.

- nonfree/strchr.c:
  the test is buggy, the behavior was undefined