posix: Use <support/check.h> facilities in tst-truncate and tst-truncate64

Remove local FAIL macro in favor to FAIL_RET from <support/check.h>,
which provides equivalent reporting, with the name of the file of the
failure site additionally included, for the tst-truncate-common core
shared between the tst-truncate and tst-truncate64 tests.

Reviewed-by: DJ Delorie <dj@redhat.com>