test/ipsec: fix test suite setup

test/ipsec: fix test suite setup

Check for valid crypto_null device before continuing.
Use valid_dev instead of valid_devs[].
Replace valid_dev_count with valid_dev_found
Call create_crypto_session for one driver only.
Refactor code so that driver capabilities can be checked in
the testsuite_setup function.

Fixes: 05fe65eb66b2 ("test/ipsec: introduce functional test")
Signed-off-by: Bernard Iremonger <bernard.iremonger@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>