A propType error was showing up during e2e tests with a `testDev`
build. It was caused by `process.env.IN_TEST` being treated as a
boolean, when in fact it is either the string `'true'` or a boolean.
`IN_TEST` has been updated to always be a boolean. `loose-envify` has
no trouble injecting boolean values, so there's no reason to treat this
as a string.