Update MissingRootLoggerTest to use test rule.

  - Added documentation about writing unit tests.
  - Added documentation for the MissingRootLoggerTest.
  - Updated said test with some better assert messages.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1581693 13f79535-47bb-0310-9956-ffa450edef68