build: use _add_swift_target_library_single

This library is meant to run on the platform where the testing occurs,
requiring bits to run.  This makes this library a target library.

Author: compnerd

test/CMakeLists.txt

@@ -230,7 +230,7 @@ foreach(SDK ${SWIFT_SDKS})
 __declspec(dllexport)
 #endif
 _Block_release(void) { }\n")
-    _add_swift_host_library_single(
+    _add_swift_target_library_single(
       BlocksRuntimeStub${VARIANT_SUFFIX}
       BlocksRuntimeStub
       SHARED