Add JS functional tests for modal example, including validation phase

Looks like the patch applies properly.

After applying this patch, go to this path (with JS turned on): examples/fapi-example/modal-form

Submit a title. You'll see that there is no submit handler message. However, if you reload the patch, you see the submit handler message.

Then, if you go to the main page for this module: examples/fapi-example

Click on the 'Modal Form' link and submit a title. You'll see the submit handler message as it should be on the page. Now, if you click the Modal Form link again, you'll see the status message again in the UI modal form.

I might have just filed a duplicate issue for this here: #2883725: fapi_example modal dialog does not show result message w/ JS

Please mark it dupe if it is. Thanks.

Also, we'll need a test for this. I think it might end up being a javascript functional test.

Postponed on #2838276: Modal Form Example not working

#2838276: Modal Form Example not working gives us a basic behavior, so now we can test and fix the validation phase.

Working on this.

Moving things around, working on the JS functional test. There are some problems with the test system: #2831506: Minimal profile disallows modal AJAX tests under JavascriptTestBase

This will fail.

This should be it.

There are todos about #2831506: Minimal profile disallows modal AJAX tests under JavascriptTestBase

Fix-O-Rama.

Now we have functional testing of the modal form.

This specific example should maybe be in the AJAX example, or maybe adding @see type stuff in the docblocks and human-readable parts.