Don't call require("mock-modules").register("test/all", ...).

The "test/all" module will never be mocked, nor should it ever need to be
reset by `dumpCache`.
Abandon <iframe> test isolation hack now that we have dumpCache.

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.