Fix headless FontManager leak

[maxkatz6]

What does the pull request do?

For each test executed we recreate application services, isolating them. Including font manager. But we do not dispose these, keeping all the native memory alive.

There are really two options to solve it:

1. Do explicit dispose (done in this PR). I went this direction, because we already did it for UnitTestApplication exactly the same way.
2. Make sure each skia object has a finalizer, and somehow break managed holds native, native holds managed reference loop in GlyphTypefaceImpl.Face GetTable delegate.

While #2 sounds reasonable to do, FontManager is practically a singleton in normal (non-unit test) applications, and won't do much there. And we still have to properly dispose AvaloniaLocator services after we don't use them.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

I hope LeakTests won't be flaky one. But also can't use dotMemory extensions properly there, as FontManager is allocated and destroyed outside of the test method itself.

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0058744-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]