Fix elf/tst-env-setuid[-static] if test needs to be rerun.

If /tmp is mounted nosuid and make xcheck is run,
then tst-env-setuid fails UNSUPPORTED with "SGID failed: GID and EGID match"
and /var/tmp/tst-sonamemove-runmod1.so.profile is created.

If you then try to rerun the test with a suid mounted test-dir
(the SGID binary is created in test-dir which defaults to /tmp)
with something like that:
make tst-env-setuid-ENV="TMPDIR=..." t=elf/tst-env-setuid test
the test fails as the LD_PROFILE output file is still available
from the previous run.

Thus this patch removes the LD_PROFILE output file in parent
before spawning the SGID binary.

Even if LD_PROFILE is not supported anymore in static binaries,
use a different library and thus output file for tst-env-setuid
and tst-env-setuid-static in order to not interfere if both
tests are run in parallel.

Furthermore the checks in test_child are now more verbose.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>