kunit: Add module attribute

Add module attribute to the test attribute API. This attribute stores the
module name associated with the test using KBUILD_MODNAME.

The name of a test suite and the module name often do not match. A
reference to the module name associated with the suite could be extremely
helpful in running tests as modules without needing to check the codebase.

This attribute will be printed for each suite.

Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Rae Moar <rmoar@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>