Re-enable the leak detection in MTRTestCase, now that the test leaks are fixed.

Author: bzbarsky-apple

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

@@ -27,15 +27,13 @@ @implementation MTRTestCase
  */
 - (void)tearDown
 {
-#if 0
 #if defined(ENABLE_LEAK_DETECTION) && ENABLE_LEAK_DETECTION
     if (_detectLeaks) {
         int pid = getpid();
         __auto_type * cmd = [NSString stringWithFormat:@"leaks %d", pid];
         int ret = system(cmd.UTF8String);
         XCTAssertEqual(ret, 0, "LEAKS DETECTED");
     }
-#endif
 #endif
 
     [super tearDown];