Even when the running kernel does not support selftests, make subtest
enumeration list known kselftests. The list is generated using
selftest listing headers copied from the kernel.

If the running kernel gains new selftest subtests, they are listed
even without copying the headers over and rebuilding IGT.

v2: Use correct names for the testcases

Signed-off-by: Petri Latvala <petri.latvala@intel.com>