Make swift-testing installable with --install-swift-testing rather than --install-swift-testing-macros (#76840)

Author: marcprux

utils/swift_build_support/swift_build_support/products/swift_testing.py

@@ -51,7 +51,7 @@ def should_test(self, host_target):
         return False
 
     def should_install(self, host_target):
-        return self.args.install_swift_testing_macros
+        return self.args.install_swift_testing
 
     def _cmake_product(self, host_target):
         build_root = os.path.dirname(self.build_dir)