Always try to destroy the binary during common methods
of test fixtures. This is safe if no other code in the test
attempted to destroy the binary.
Take advantage of the fact spvBinaryDestroy is a no-op on a nullptr,
by eliminating the null pointer check in the caller.
Start using GMock: modify CMakeLists, fix googletest URL in readme.
Add useful utilities to the TestFixture class. Also make it conform to
go/gunit recommendations about setup/teardown.
