Improve detection of Test Targets

[AliSoftware]

Since #169 has been merged, we can customize the name of the Test Target and choose whatever we like 👍

But that is not consistant with how XcodeprojHelper#test_targets determines the list of test targets 😢

We should either:

• Use the test_target_name specified during the config to inject it in this list
• Use a better way to detect which targets are Test Targets, for example check which targets have xctest for their WRAPPER_EXTENSION build setting.
• Use Xcodeproj to list PBXNativeTarget objects whose productType is com.apple.product-type.bundle.unit-test