Enable verbose logging for tests

[jessiezcc]

Came up from code review. Christie suggested on leveraging gLog for log level control

Here is the plan:

• introduce a new tag to turn on verbose logging
• switch from log to glog
• we can either provide verbose logging as a static option (i.e. treating 10 as verbose), or open up leveling so that test owners can specify whatever log level they deem proper, and when tests are run, we will specify a numeric number to filter logs. For the latter, I'm concerned about consistency, while former lacks the flexibility.
• update documentation on test loggging