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

commit 2c6ea4e2 upstream.

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>
Signed-off-by: Liwei Song <liwei.song@windriver.com>