tests: shell: add feature probe for sets with more than one element

tests: shell: add feature probe for sets with more than one element

Kernels < 5.11 can handle only one expression per element, e.g.
its possible to attach a counter per key, or a rate limiter,
or a quota, but not two at the same time.

Add a probe file and skip the relevant tests if the feature is absent.

Signed-off-by: Florian Westphal <fw@strlen.de>