Test action macroExpansion allows unavailable buildable reference.

[erneestoc]

Relates to: https://github.com/yonaskolb/XcodeGen/pull/1468

Since the buildableReference passed to the test action macroExpansion param is literally the first buildable reference, there's a chance that buildable reference is not part of the test buildable references, so "Expand Variables Based On" defaults to None (for the test action)

A potential solution is to use the first buildable reference from testBuildTargetEntries instead if the buildableReference is not part of (testBuildTargetEntries).

[erneestoc]

done!