Instead of checking in binary .xpi files, we now have the source files
under source control and generate the .xpi as part of the build.
The build system mechanism to generate the .xpi files is hacky. But it's
similar to what existing processes use. Bug 988938 will eventually make
this more sane.
Since the produced .xpi files no longer have stable content, the SHA-1s
of the experiments are now calculated at test run time.
