Fix codemirror style for TestGenerator

[jhen0409]

Before

After

[jhen0409]

But it doesn't completely solve the problem, if Inspector layout is Up-Down mode:

So I make alexkuz/redux-devtools-inspector#46.

[zalmoxisus]

@jhen0409, thanks a lot for working on this! The funny part is that I was using the same workaround (and yes it worked only in the first case), but removed it in ac998cc as for some reason it was freezing the browser when the window is too small (strangely this bug appeared with Chorome 42).

Better to address it in the Inspector Monitor, making the inner part scrollable and the toolbar fixed on the top, which is a problem not only for custom components.

[zalmoxisus]

It still hangs the browser in the latest Chrome, though fixed in Canary. To reproduce, make the window small and click the Test tab.

[jhen0409]

@zalmoxisus, ok. :D Just checkout alexkuz/redux-devtools-inspector#46, it should can be solved the problem.