Add unit tests for OomWatcher that actually test the logic defined in
the `Start` method. As a result of an earlier refactor, its now trivial
to mock the OOMInstance events which the `oom_watcher` is supposed to be
watching.
This diff contains a strict refactor; there are no behavioral changes.
Address a long standing TODO in `oom_watcher_linux_test.go` around test
coverage. We refactor our `oom.Watcher` so it takes in a struct
fulfulling the `streamer` interface (i.e. defines `StreamOoms` method).
In production, we will continue to use the `oomparser` from `cadvisor`.
However, for testing purposes, we can now create our own `fakeStreamer`,
and control how it streams `oomparser.OomInstance`. With this fake, we
can implement richer unit testing for the `oom.Watcher` itself.
Actually adding the additional unit tests will come in a later commit.
The `recorder.PastEventf` method wasn't actually working as advertised.
It was supposed to accept a timestamp, which would be used when
generating the event. However, as the
[source code](https://github.com/kubernetes/kubernetes/blob/master/staging/src/k8s.io/client-go/tools/record/event.go#L316)
shows, this `timestamp` was never actually used.
In other words, `PastEventf` is identical to `Eventf`.
We have two options: one would be to fix `PastEventf` so that it works
as advertised. The other would be to delete `PastEventf` and only
support `Eventf`.
Ultimately, I could only find one use of `PastEventf` in the code base,
so I propose we just delete `PastEventf` and convert all uses to
`Eventf`.
