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-class-properties/test/fixtures/loose/foobar/options.json

@@ -1,4 +1,4 @@
 {
-  "plugins": ["external-helpers"],
-  "presets": ["stage-0", "es2015"]
+  "plugins": ["external-helpers", ["transform-class-properties", {"loose": true}]],
+  "presets": ["es2015"]
 }