Define an abstract DiffAlgorithm test framework

Define an abstract DiffAlgorithm test framework

For certain tiny input sequences, every DiffAlgorithm should produce
exactly the same results, as there should be no ambiguity.  Package
these up in an abstract TestCase that algorithms can extend from in
order to perform basic validation of their implementation.

Since these tests are more complete than what we used to have for
the MyersDiff algorithm, throw away Johannes' tests and only use
this new package.

Change-Id: I9a044330887c849ad4c78aa5c7aa04c783c10252
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>