Refactorings to reef-tests

This PR doesn't solve any specific issue, but instead does some code improvements to the REEF tests

  * Most tests now have a one-liner in the class JavaDoc as well as in `package-info.java`
  * Repetetive code (such as a driver start handler that requests a single evaluator) is moved to `c.m.reef.tests.library`
  * Most tests have had theit job renamed to start with `TEST_`
  * Classes with generic names such as `Driver` or `Task` have been renamed.

 Again, this PR contains *no functional changes*.