Add parameter to bunit template, that chooses the underlying test framework

[egil]

Instead of creating different templates, one for bUnit with xUnt, one for bunit with Nunit, etc., I think it is better to have one template, but instead add a (mandatory?) parameter, that picks the underlying test framework.

E.g.:

• dotnet new bunit -o my-test-project -framework xunit
• dotnet new bunit -o my-test-project -framework nunit
• dotnet new bunit -o my-test-project -framework mstest

This should be done in when #128 and #127 is done.

[denisekart]

This looks like an interesting issue. I'd like to give it a shot.

(I'll probably have some questions along the way)

[egil]

That would be great. I cannot help much with how to get the bunit.template arguments working, but can help with what should be generated.

A few notes:

1. Look to https://bunit.dev/docs/getting-started/writing-tests.html for inspiration on what should be in each template project type (e.g. BunitTestContext for NUnit and MSTest)
2. We also need to update the verification.yaml workflow such that it tests all supported arguments to the template