selftests/x86/xstate: Consolidate test invocations into a single entry

Currently, each of the three xstate tests runs as a separate invocation,
requiring the xstate number to be passed and state information to be
reconstructed repeatedly. This approach arose from their individual and
isolated development, but now it makes sense to unify them.

Introduce a wrapper function that first verifies feature availability
from the kernel and constructs the necessary state information once. The
wrapper then sequentially invokes all tests to ensure consistent
execution.

Update the AMX test to use this unified invocation.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250226010731.2456-8-chang.seok.bae@intel.com