thunderbolt: Add KUnit tests for path walking

This adds KUnit tests for path walking which is only dependent on
software structures, so no hardware is needed to run these.

We make these available only when both KUnit and the driver itself are
built into the kernel image. The reason for this is that KUnit adds its
own module_init() call in kunit_test_suite() which generates linker
error because the driver does the same in nhi.c. This should be fine for
now because these tests are only meant to run by developers anyway.

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