Updated testing docs for 1.x

[chiedo]

I put together some updated testing docs for React-router 1.x.

There wasn't much to write as testing should "just work" with react-router 1.x but mainly this doc helps those upgrading from prior testing setups.

As I do more testing and run into issues, I can add more to the docs.

Re issue #2149

[chiedo]

I placed the testing docs at the same location as the old testing docs so that the links already existing scattered through blogposts and forums will still work.

[knowbody]

Very cool, thanks for that, mind adding an example to this guide as well?

@chiedojohn also maybe worth mentioning babel-jest and the config with webpack, what do you think?

[chiedo]

I think that's a good call! @knowbody

I mentioned babel-jest. I don't use webpack though so maybe someone else could modify submit a PR for that portion?

In regards to the example. Do you think I should add that to the examples directory and link to it? Or just create the example in testing.md?

[knowbody]

Cool, I fill the gap with webpack.

The example just put it in the bottom, something like:

Example:

some code

and this is how you can test it:

some code

[chiedo]

Example added @knowbody

[knowbody]

awesome, thank you!
one last thing can you squash the commits? and then I'll merge

[chiedo]

Squashed!