Protocol test framework improvements

[st0012]

I made these changes when debugging Ruby head's hanging issue and they did make the build failures easier to catch.

1. Both debuggee connection and workflows should have timeout
2. Rename WebSocketClient#cleanup to #close because that's all it does
3. Only perform cleanup logic when the socket/threads are present, which may not be the case when the error happens during test setup.

[st0012]

@peterzhu2118 I could also use your help on this test improvement. You can run protocol tests with bundle exec rake test_protocol, which is not included in rake test.

[peterzhu2118]

Looks good, thank you!