Add an interface between CleanStaleEntries and the lower-level
conntrack helpers (ClearEntriesForIP, etc), and a fake implementation
of that interface, so that we can explicitly test CleanStaleEntries's
logic.
Remove some comments from conntrack.go that were explaining the
functions' callers rather than explaining the functions themselves
(and which were redundant with other comments in the callers anyway).
Fix the test names to match the functions they are testing.
Abstract out the repetitive FakeExec handling.
Explicitly specify the "expectCommand" in each one, to make it clearer
that that's really the part that we're testing.
For everything except TestExec(), test each case with both a "success"
result and a "nothing to delete" result from the conntrack binary.
