unit tests don't need APPLICATION_EXTENSION_API_ONLY set to yes; in fact, doing so causes this compiler warning in Xcode 9: ld: warning: linking against a dylib which is not safe for use in application extensions: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/iphonesimulator/libswiftXCTest.dylib