Formatter: Add a test for multi-line ObjC dict literals.

As it turns out, this already works reasonably well.

This example from http://clang.llvm.org/docs/ObjectiveCLiterals.html
NSDictionary *dictionary = @{
    @"name" : NSUserName(),
    @"date" : [NSDate date],
    @"processInfo" : [NSProcessInfo processInfo]
};

is formatted like
NSDictionary *dictionary = @{ @"name" : NSUserName(), @"date" : [NSDate date],
                              @"processInfo" : [NSProcessInfo processInfo] };

There's already a FIXME in NestedStaticInitializers about supporting one
initializer per line, which is really all that's missing here too.