[corlib/I18N] Various EncoderFallback fixes

[akoeplinger]

[I18N] Fix wrong charIndex in encoder fallback
We need to modify charIndex/charCount _after_ we called
the fallback encoder otherwise it'll get the wrong index.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=45893

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[corlib] Use Latin1Encoding from referencesource
Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=53296

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[corlib] Add tests for EncoderFallback
Also remove some debug spew.

[corlib] Add corefx System.Text.Encoding tests
This adds about 9800 Encoding related tests from corefx, all passing after the latest fixes.

[I18N] Add more encoding tests
There was very little test coverage of the I18N encodings before.

This adds a bunch of tests based on Latin1EncodingTest.cs since latin1 is quite similar to these encodings.

A few of the added tests fail in some encodings due to what looks like bugs in the implementation, disabled those temporarily, see FIXME's. (filed https://bugzilla.xamarin.com/show_bug.cgi?id=60216 for tracking)