Fix unit test.

Author: bzbarsky-apple

src/darwin/Framework/CHIPTests/MTRPerControllerStorageTests.m

@@ -1043,7 +1043,7 @@ - (void)test007_TestMultipleControllers
 // startControllerWithRootKeys into a test helper.
 - (void)testControllerServer
 {
-    __auto_type queue = dispatch_get_main_queue();
+    __auto_type queue = dispatch_queue_create("testControllerServer.queue", DISPATCH_QUEUE_SERIAL);
 
     __auto_type * rootKeys = [[MTRTestKeys alloc] init];
     XCTAssertNotNil(rootKeys);