Bump test target to 10.15 because Xcode expects that

config/Tests-Shared.xcconfig

@@ -17,3 +17,6 @@ OTHER_SWIFT_FLAGS = $(inherited) -Xfrontend -debug-time-function-bodies
 
 // Disable for tests. https://twitter.com/krzyzanowskim/status/1191515868401983489
 BUILD_LIBRARY_FOR_DISTRIBUTION = NO
+
+MACOSX_DEPLOYMENT_TARGET = 10.15
+