libassuan: add ptest support

libassuan: add ptest support

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

The implementation installs test programs (version, pipeconnect,
fdpassing) and the shell script test (fdpassing-socket.sh).

All 4 tests pass on qemux86-64:
START: ptest-runner
BEGIN: /usr/lib/libassuan/ptest
PASS: version
Received data `Your lucky number is 3552664958674928.  Watch for it everywhere.'
PASS: pipeconnect
PASS: fdpassing
PASS: fdpassing-socket.sh
END: /usr/lib/libassuan/ptest
STOP: ptest-runner
TOTAL: 1 FAIL: 0

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