Add a test-only clearLog command for clearing out RamLog, use it in tests when checking for a specific log message

Some tests call the getLog command to try to wait for a certain log message to show up. If that message already showed up previously, however, the test can move on prematurely before the log message is actually generated at the point they want. Adding a clearLog command would enable such tests to clear the log before waiting for the message they are expecting to appear.