bsdiff and bspatch are in C and had no unittest. This patch keeps
those programs as C code, but adds the C++ gtest unittest framework
to allow testing them. Two simple unittests added to validate that the
unittests work.

BUG=None
TEST=make test; ./unittests

Change-Id: I8bca6b0c6bc5d5880464183d50a602c9886d20d0