[TACHYON-217] Add further documentation and unit tests

Initial attempt to add more documentation and tests for open delete and delete non-existing. These files make use of the eclipse formatting xml but used in IntelliJ hence some formatting differences may exist.
"This test should return true" since the test returns void, this statement feels wrong. Can we drop it?

Also, don't think we have to document that the test should pass without exceptions, since junit works that way.