Add test coverage for setting target configuration

One of the ways the bug in #2979882: "The machine-readable name is already in use" error when adding multiple source keys can be reproduced, is in the following way:

1. Add a target
2. From the sources select list, select "New custom source.." and type in a source value.
3. Configure target configuration.

So to write a test for the bug, it would be good to involve setting target configuration for it. There's however no test coverage for this action yet and it is stated as @todo in FeedsJavascriptTestBase::addMappings().

So let's add that. And while we are at it, break down FeedsJavascriptTestBase::addMappings() into multiple methods so the other issue could more easily do steps on the mapping form in a different order. Because the bug from the other issue is not exposed when setting target configuration first and set custom sources after.

Patch will follow.