[lit] Remove dead code not referenced in the LLVM SVN repo.

Summary:
This change removes the intermediate 'FileBasedTest' format from lit. This
format is only ever used by the ShTest format, so the logic can be moved into
ShTest directly.

In order to better clarify what the TestFormat subclasses do, I fleshed out the
TestFormat base class with Python's notion of abstract methods, using
@abc.abstractmethod. This gives a convenient way to document the expected
interface, without the risk of instantiating an abstract class (that's what
ABCMeta does -- it raises an exception if you try to instantiate a class which
has abstract methods, but not if you instantiate a subclass that implements
them).

Reviewers: zturner, modocache

Subscribers: sanjoy, llvm-commits

Differential Revision: https://reviews.llvm.org/D34792

llvm-svn: 306623