thunderbolt: Allow KUnit tests to be built also when CONFIG_USB4=m

This adds a bit more build coverage for the tests even though these are
not expected to be enabled by normal users and distros. In order to make
this working we need to open-code kunit_test_suite() and call the
relevant functions directly in the driver init/exit hook.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>