At the moment any command line option handling done by tests will
interfere with the option handling of the subtest interface. To fix this
add a new version of the subtest_init function accepting optional short
and long command line options. Merge these together with the subtest
interface's own long options and handle both together in the same
getopt_long call.
Signed-off-by: Imre Deak <imre.deak@intel.com>