[Darwin] MTRMockCB reference in MTRTestCase should not be behind HAVE_NSTASK (#38004)

Author: jtung-apple

src/darwin/Framework/CHIPTests/TestHelpers/MTRTestCase.mm

@@ -27,8 +27,6 @@
 // Tasks that are not scoped to a specific test, but rather to a specific test suite.
 static NSMutableSet<NSTask *> * sRunningCrossTestTasks;
 
-static MTRMockCB * sMockCB;
-
 static void ClearTaskSet(NSMutableSet<NSTask *> * __strong & tasks)
 {
     for (NSTask * task in tasks) {
@@ -39,6 +37,8 @@ static void ClearTaskSet(NSMutableSet<NSTask *> * __strong & tasks)
 }
 #endif // HAVE_NSTASK
 
+static MTRMockCB * sMockCB;
+
 @implementation MTRTestCase {
 #if HAVE_NSTASK
     NSMutableSet<NSTask *> * _runningTasks;