[IntegrationSpec] Compare execution_output.txt

@mrackwitz now that the downloader doesn't have caches, I can think of no reason to not also compare the execution output.

\c @alloy @kylef 
`hg` seems appropriate, because we have at least one integration spec, which uses a Mercury transport, but `svn` would be IMHO premature generalization. `chomp` should work, as we know that we want to remove always the trailing newline character, I guess that `which` is pretty stable.