Add a specific test for the crash reported by Phil Sutter and addressed
in the previous patch. The test cases that, in my intention, should
have covered these cases, that is, the ones from the 'concurrency'
section, don't run these sequences tightly enough and spectacularly
failed to catch this.

While at it, define a convenient way to add these kind of tests, by
adding a "reported issues" test section.

It's more convenient, for this particular test, to execute the set
setup in its own function. However, future test cases like this one
might need to call setup functions, and will typically need no tools
other than nft, so allow for this in check_tools().

The original form of the reproducer used here was provided by Phil.
Reported-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>