test/run_tests.pl: don't mask test failures.

Switch to TAP::Harness inadvertently masked test failures.
Test::Harness::runtests was terminating with non-zero exit code in case
of failure[s], while TAP::Harness apparently holds caller responsible
for doing so.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>