test: add skipped return result

test: add skipped return result

This commit allows a test to return "skipped", indicating
that it cannot be run. This is useful for PMDs which have
not been compiled due to the unavailability of dependencies,
or their explicit disabling in the build configuration.

The result printing is updated to correctly indicate if a
test has been skipped.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>