This was not necessary when we were running each test in its own
`<iframe>`, and it doesn't seem to affect any test behavior currently, but
it seems wise for the sake of test isolation and hygiene.
This cuts the running time of `grunt phantom:run` from 4.4s to 3.1s on my
machine, because we no longer have to load/execute a separate instance of
`react-test.js` in a separate `<iframe>` for each test.
We're using populist for building the bundle of test modules and their
dependencies, so it seems worthwhile for consistency to do the same for
the test harness.
Ben Newman