Add better support for custom test runners.

Summary:
I finally got around to merging the many, many changes to lit.cfg into
Android's libc++. This patch makes it simpler to actually use a custom
configuration and test format.

First, I've factored out _build, _run, and _clean methods from
_execute_test, since these are the likely parts that will need to be
overridden. This is likely a first step in the work jroelofs has been
doing with improving cross-compiling test execution.

Second, I've added a `configuration_variant` to the config. This
entry, if present, is a string that forms the prefix of the class that
is to be used to configure the test runner. For example, Android sets
`config.configuration_variant = 'Android'`, and this causes an object
of type `AndroidConfiguration` to be constructed.

As an example of how this will be used, see:
https://android-review.googlesource.com/#/c/116022/

Reviewers: jroelofs, mclow.lists, EricWF

Reviewed By: EricWF

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6373