tests: shell: add crash reproducer

tests: shell: add crash reproducer

Two reports point to a crash in nft when 'flush' is provided
on existing ruleset.  In that case, nft will crash with a null-ptr
dereference.

"evaluate: do not inconditionally update cache from flush command"
causes the commit to fail due to a cache inconsistency, we then trip
over NULL location->indesc.  Cause of 2nd bug not known yet, not sure
how to fix cache issue either, so only adding reproducer so this can be
fixed later.

Without erec bug, the (errnoeous) error message would be
Could not process rule: File exists

Reported-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Reported-by: Timothy Redaelli <tredaelli@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>