tests/shell: honor NFT_TEST_FAIL_ON_SKIP variable to fail on any skipped tests

tests/shell: honor NFT_TEST_FAIL_ON_SKIP variable to fail on any skipped tests

The test suite should pass with various kernels and build
configurations. Of course, that means, that some tests will be
gracefully skipped, and we don't treat that as an overall failure.

However, it should be possible to run a specific kernel (net-next?) and
build configuration, where we expect that all tests pass.

Add an option to fail the run, if any tests were skipped. This is to
ensure that we don't have broken tests that never pass.

This will make more sense with automated CI is running, to enable on a
test system and ensure that at least on that system, all tests pass.

Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>