Make XUnit2TestGeneratorProvider.SetTestMethod virtual

[JoshKeegan]

Make XUnit2TestGeneratorProvider.SetTestMethod virtual so that it can be replaced by generator plugins. I needed to make this change for a new feature in my generator plugin, src using it here: https://github.com/JoshKeegan/xRetry/blob/5494809b736bc50c8bcdf24ece71b6623a2f6d4c/src/xRetry.SpecFlow/TestGeneratorProvider.cs#L35

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue).
• [x] New feature (non-breaking change which adds functionality).
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected).
• [ ] Performance improvement
• [ ] Refactoring (so no functional change)
• [ ] Other (docs, build config, etc)

Checklist:

• [ ] I've added tests for my code. (most of the time mandatory)
• [x] I have added an entry to the changelog. (mandatory)
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.

[SabotageAndi]

Sure, no problem to make this virtual.

[SabotageAndi]

@JoshKeegan this change is released to nuget.org

[JoshKeegan]

@JoshKeegan this change is released to nuget.org

Thanks!