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.
