Support keys and configs in SplitClientForTest

[GreyTeardrop]

SplitClientForTest can be useful for unit-testing code that depends on SplitClient, especially in dynamic scenarios where a static config per test class won't work. As of now, SplitClientForTest doesn't support customization of split keys or configs - it always returns the same treatment for all keys of a feature flag and always returns null as a split config.

This change extends SplitClientForTest, so it's possible to set treatments for each feature flag and key, and also possible to set split config in addition to the treatment name.

[GreyTeardrop]

@chillaq is this a change you might be interested in reviewing?