Don't merge test options. (#6157)

* Don't merge test options.

Particularly, I don't want `lodash/merge` to merge my specific plugins
with the general test plugins. It led to odd behavior where I could
enable a loose transform in my specific test, just to have it overridden
by the test fixture's general options.

* Need options

packages/babel-plugin-transform-async-to-module-method/test/fixtures/bluebird-coroutines/class/options.json

@@ -1,3 +0,0 @@
-{
-  "plugins": ["external-helpers"]
-}