Add test covering incorrect application of avar table

[maxmelander]

Adds test to cover the issue where the avar table was beeing applied to the same coordinate multiple times when calling set_variations ( #136 ).

This test will pass when harfbuzz/ttf-parser@1cac7c7 is taken into use.

[RazrFalcon]

I have updated ttf-parser, so the test should not fail anymore.
But since we auto-generate tests, there are a couple more steps needed.

1. Put your test data into tests/custom/bugs.tests.
2. Move the font to tests/fonts/rb_custom.
3. Move variations_006 to tests/shaping/custom.rs as bugs_002.

That should be enough.

[RazrFalcon]

Thanks!