scripts/oe-selftest: add command line option to list test classes

While trying to discover what tests are available, I felt the
need to be able to list all individual tests so I can run specific
tests.

This patch adds the "--list-classes" command line option that
lists the unit test classes and methods available.

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>