tests: use virfirewallmock instead of hasNetfilterTools

tests: use virfirewallmock instead of hasNetfilterTools

Instead of checking for specific error that the binaries are not
available mock the virFindFileInPath function. This way we don't have
to skip these tests on host where the binaries are missing.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>