zaptest: Provide verifying logger to notice unexpected logs

[prashantv]

Want something to provide functionality similar to tchannel-go, a logger which will fail when it sees unexpected log messages.

Ideally, we'd avoid requiring a defer by:

• Create a logger with a set of options for what is allowed ideally using functional options that allow you to do things like IgnoreMessage("message").AnyTimes(), or specify minimum levels etc
• Store all logs in-memory, until it receives an unexpected log message
• Once we hit a bad message, fail the test, log all previous messages (probably using t.Log), and then start logging all further messages to the logger.

[WGH-]

This doesn't satisfy all the requirements you mentioned, but it's still "a logger which will fail when it sees unexpected log messages" implemented with just a little bit of boilerplate:

core, observedLogs := observer.New(zap.WarnLevel)
defer func() {
	assert.Empty(t, observedLogs.TakeAll())
}()
logger := zap.New(core)