nptl: Convert tst-sem11 and tst-sem12 tests to use the test driver

Fix an issue with commit 2af4e3e5 ("Test of semaphores.") by making
the tst-sem11 and tst-sem12 tests use the test driver, preventing them
from ever causing testing to hang forever and never complete, such as
currently happening with the 'mips-linux-gnu' (o32 ABI) target.  Adjust
the name of the PREPARE macro, which clashes with the interpretation of
its presence by the test driver, by using a TF_ prefix in reference to
the name of the 'tf' function.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>