oe-selftest: Improved --list-classes when determining test names

oe-selftest: Improved --list-classes when determining test names

--list-classes does a weak validation when determining test names:
(if method.startswith("test_") which could report any class attribute
that starts with 'test_' as a valid test case.

This fix checks that the class attribute that starts with 'test_'
is also callable (is a method).

Fix for [YOCTO #8862]

(From OE-Core rev: 175810503d5596370cf7d840539ebdf35cf30278)

Signed-off-by: Daniel Istrate <daniel.alexandrux.istrate@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>