selftests: harness: Restore order of test functions

selftests: harness: Restore order of test functions

The recent addition of explicit constructor orders for fixture tests
broke the ordering of those relative to non-fixture tests and the
reverse-constructor-order detection.

Restore the ordering of the test functions relative to each other by
using the same explicit test order for all test registrations and
__constructor_order_first().

Rename the constant, as it is not specific to TEST_F() anymore.

Link: https://lore.kernel.org/r/20260422-kselftests-harness-order-v2-1-93ea980ea3ac@linutronix.de
Fixes: 6be268151426 ("selftests/harness: order TEST_F and XFAIL_ADD constructors")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>