selftests/kselftest_harness: Add harness-selftest.expected to TEST_FILES

The harness-selftest.expected is not installed in INSTALL_PATH.
Attempting to execute harness-selftest.sh shows warning:

diff: ./kselftest_harness/harness-selftest.expected: No such file or
directory

Add harness-selftest.expected to TEST_FILES.

Link: https://lore.kernel.org/r/20250909082619.584470-1-yi1.lai@intel.com


Fixes: df82ffc5 ("selftests: harness: Add kselftest harness selftest")
Signed-off-by: Yi Lai <yi1.lai@intel.com>
Reviewed-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>