Add explicit self

Author: ncooke3

FirebaseRemoteConfig/Tests/Unit/RCNRemoteConfigTest.m

@@ -1916,7 +1916,7 @@ - (void)testSetCustomSignalsMultipleTimes {
     [_configInstances[i] setCustomSignals:testSignals1
                            withCompletion:^(NSError *_Nullable error) {
                              XCTAssertNil(error);
-                             [_configInstances[i]
+                             [self->_configInstances[i]
                                  setCustomSignals:testSignals2
                                    withCompletion:^(NSError *_Nullable error) {
                                      XCTAssertNil(error);