[ios] Silence MGLCalloutView related warnings in tests (#6231)

The annotation view test uses a fake MGLTestCalloutView that
declares conformance to the MGLCalloutView protocol. However,
several properties and a method were not implemented in the test
which caused several warnings that were visible if you compiled
the SDK tests. This change stubs out the properties and method
so the warnings go away.

platform/ios/test/MGLAnnotationViewTests.m

@@ -23,6 +23,10 @@ static NSString * const MGLTestAnnotationReuseIdentifer = @"MGLTestAnnotationReu
 
 @interface MGLTestCalloutView: UIView<MGLCalloutView>
 @property (nonatomic) BOOL didCallDismissCalloutAnimated;
+@property (nonatomic, strong) id <MGLAnnotation> representedObject;
+@property (nonatomic, strong) UIView *leftAccessoryView;
+@property (nonatomic, strong) UIView *rightAccessoryView;
+@property (nonatomic, weak) id<MGLCalloutViewDelegate> delegate;
 @end
 
 @implementation MGLTestCalloutView
@@ -32,6 +36,8 @@ static NSString * const MGLTestAnnotationReuseIdentifer = @"MGLTestAnnotationReu
     _didCallDismissCalloutAnimated = YES;
 }
 
+- (void)presentCalloutFromRect:(CGRect)rect inView:(UIView *)view constrainedToView:(UIView *)constrainedView animated:(BOOL)animated { }
+
 @end
 
 @interface MGLAnnotationViewTests : XCTestCase <MGLMapViewDelegate>