qa: Add skip_if_no_lsof_on_nonlinux helper and use it where needed

Some functional tests on non-Linux platforms rely on the `lsof` utility.
However, we treat all other functional test dependencies, such as
additional Python modules, as optional, and skip dependent tests if
those are unavailable.

This change makes `lsof` optional as well.

test/functional/feature_bind_extra.py

@@ -33,6 +33,7 @@ class BindExtraTest(BitcoinTestFramework):
 
     def skip_test_if_missing_module(self):
         self.skip_if_platform_not_posix()
+        self.skip_if_no_lsof_on_nonlinux()
 
     def setup_network(self):
         loopback_ipv4 = addr_to_hex("127.0.0.1")