gnutls: add ptest support

Add gnutls unit-tests into ptest. Only self-contained
tests (no external file system dependencies) are included.

Some tests have been put in a disallow list in the run-ptest;
these tests either require additional files (certificates) or
environment setup prior to being run, via wrapper shell script.
As autotools packages are built out-of-tree, it is cumbersome
to add additional files. With regards to tests requiring wrapper
scripts, it is possible to add logic to run these tests under
run-ptests but that will require more effort to maintain, hence
skipped.

Verified via:
$ runqemu qemux86-64 kvm nographic serial qemuparams='-m 4096 -smp 32'
root@qemux86-64:~# ptest-runner gnutls
START: ptest-runner
BEGIN: /usr/lib64/gnutls/ptest
PASS: aead-cipher-vec
PASS: alerts
PASS: anti_replay
...
gnutls test summary:
--------------------
total: 375
pass : 364
fail : 0
skip : 11

DURATION: 184
END: /usr/lib64/gnutls/ptest
2023-02-01T13:55
STOP: ptest-runner
TOTAL: 1 FAIL: 0

Signed-off-by: Ravineet Singh <ravineet.a.singh@est.tech>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>