Support Build Settings of Test Targets

Unit Test Bundles need specific build settings and are not covered by the build settings in the constants so far. That would be helpful for changes like that CocoaPods/CocoaPods#2975, which need to cover the target type `:unit_test_bundle`.

**:warning: Warning**:
It would make sense to decide on the further concept for retrieving build settings. (Deserializing stored xcconfigs, parsing Xcode's templates or keep them in `Constants::COMMON_BUILD_SETTINGS`)