selftests: netfilter: skip br_netfilter queue tests if kernel is tainted

These scripts fail if the kernel is tainted which leads to wrong test
failure reports in CI environments when an unrelated test triggers some
splat.

Check taint state at start of script and SKIP if its already dodgy.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>