Eradicate Nunit

[psfinaki]

Closes #13654

Nunit is dead, long live xUnit!

See comments.

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Notice that the total amount of tests displayed in ADO is a bit different. This seems to be due to different interpretations of Xunit's MemberData.

Nunit:

Xunit:

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The tests should be compared within the same adapter, so first of all, a sanity check using VS adapter:

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Now, let's look at where the numbers differ in the CI:

Note these 3 projects are the only ones having MemberData tests.

Different UIs (loggers and adapters) count this at different points and in different times.

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Take BreakpointResolutionServiceTests for example. This is a MemberData theory with 7 cases.

Notice that VS marks this 1 test with 7 outcomes.

Nunit will not show this at all:

Unless you search the exact test by name:

Xunit shows this even when searching by file name:

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Now take another MemberData theory, Unmatched parentheses in RemoveUnnecessaryParenthesesTests:

This MemberData theory is created via CE and so VS shows this as 14 distinct tests.

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Now, Xunit and NUnit ADO UIs imply different numbers here as well. What matter is the number of tests in the internal XML format though, and these are the same, I checked:

So this is all a bit complicated but hopefully this makes enough evidence that tests are executed alright.

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