[Infra] Suppress designated initializer warning in FIRQueryFake (#15123)

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

FirebaseTestingSupport/Firestore/Sources/FIRQueryFake.mm

@@ -16,10 +16,13 @@
 
 @implementation FIRQueryFake
 
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wobjc-designated-initializers"
 - (instancetype)init {
   // The object is partially initialized. Make sure the methods used during testing are overridden.
   return self;
 }
+#pragma clang diagnostic pop
 
 - (void)getDocumentsWithCompletion:(FIRQuerySnapshotBlock)completion {
   if (self.getDocumentsHandler) {