tests: shell: add feature check for count output change

tests: shell: add feature check for count output change

New kernels with latest nft release will print the number
of set elements allocated on the kernel side.

This causes shell test dump validation to fail in several
places.  We can't just update the affected dump files
because the test cases are also supposed to pass on current
-stable releases.

Add a feature check for this.  Dump failure can then use
sed to postprocess the stored dump file and can then call

diff a second time.

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