libksba: add ptest support

libksba: add ptest support

Add ptest support to enable automated testing of libksba
using ptest-runner.

The implementation builds and installs test executables from
the tests/ directory along with the required test data files
from the samples/ directory.

Test results on qemux86-64:
        PASS: cert-basic
	PASS: t-cms-parser
	PASS: t-crl-parser
	PASS: t-der-builder
	PASS: t-dnparser
	PASS: t-ocsp
	PASS: t-oid
	PASS: t-reader

All 8 tests pass.

Signed-off-by: Pratik Farkase <pratik.farkase@est.tech>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>