Support "expect no packets" in STF (#55)

Specifying `expect 1` as an STF statement will tell the test runner to
expect 0 packets to appear on interface 1. Any packets that egress
interface 1 will cause an error. This is useful for test cases that
explicitly should not generate packets -- e.g. for filtering programs.

docker/scripts/stf/bmv2stf.py

@@ -387,6 +387,9 @@ def do_command(self, cmd):
                 data = ''.join(data.split())
             if data != '':
                 self.expected.setdefault(interface, []).append(data)
+            else:
+                # No data provided: expect 0 packets on this intf.
+                self.expected.setdefault(interface, [])
         else:
             if self.options.verbose:
                 print("ignoring stf command:", first, cmd)