This fixes following three memory leaks in test utilities.
There is no problem in production code and this CL doesn't affect
any production behaviors.

- SkTypeface leaks due to forget calling SkSafeUnref in dtor.
- MinikinFontForTest leaks during constructing FontCollection.
- FontCollection leaks due to unnecessary AddRef.

Change-Id: I22e1e0307f1b2499296acb1aacc3ef66076a36e9