By checking that there is no more than 1 line in case.skipif and that it
is not empty, we know there is only a single line in case.skipif.

This makes sure that a Given block is available to test cases that rely
on the same Given block but follow a SkipIf block.

Add initial tests.

* Both blocks work like Execute but only for vimscript.
* The Require block skips the entire test file is some condition is met.
* The SkipIf block skips a single test and must follow its Given block.
* A special VaderSkip command is available in both blocks to perform
  the skip. Internally, a VaderSkipped exception is thrown to signal
  skips.