Capture logs in tests

[glbrntt]

Motivation:

In #902 we changed behaviour such that gRPC will not log unless you
explicitly pass in a logger. When tests fail it's useful to have the
logs to triage them, especially when they fail under CI, for example.

Modifications:

• Add a capturing log handler factory to the test module which captures
  all logs emitted by handlers it makes
• Wire up loggers in tests
• Hook up GRPCTestCase to print the captured logs only if a test fails
  (or an environment variable was set)
• Add a bunch of missing super.setUp and super.tearDown calls

Result:

• If a test fails we get all .trace level logs emitted during that
  test