[Feature] Change build settings (extra_config) in UnitTests target too

[AliSoftware]

Hi @gfontenot

I wanted to change specific Build Settings in my Unit Tests target (namely remove TEST_HOST and BUNDLE_LOADER build settings to set empty values, so that my CI don't try to launch the simulator and fail, whereas I don't have any application test but only functional ones)

Using extra_config only changes Build Settings of the App Target. It would be great to have a tests_extra_config or something similar for the Unit Tests Target too.

[gfontenot]

This is a great suggestion, thanks. This should be a quick addition.

[gfontenot]

Implementing this with #213

[AliSoftware]

👍 🎉