selftests/tpm2: Add the missing TEST_FILES assignment

The Python files required by the selftests are not packaged because of
the missing assignment to TEST_FILES. Add the assignment.

Cc: stable@vger.kernel.org
Fixes: 6ea3dfe1 ("selftests: add TPM 2.0 tests")
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Reviewed-by: Petr Vorel <pvorel@suse.cz>