selftests/rseq: Add config fragment

selftests/rseq: Add config fragment

Currently there is no config fragment for the rseq selftests but there are
a couple of configuration options which are required for running them:

 - CONFIG_RSEQ is required for obvious reasons, it is enabled by default
   but it doesn't hurt to specify it in case the user is usinsg a
   defconfig that disables it.

 - CONFIG_RSEQ_SLICE_EXTENSION is tested by the slice_test test, the
   test will fail without it.

Add a configuration fragment which enables these options, helping encourage
CI systems and people doing manual testing to run the tests with all the
features. This also requires CONFIG_EXPERT since it is a dependency for
slice extension.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20260424-selftests-rseq-config-fragment-v2-1-a9475996edcb@kernel.org