Remove implicit cast from ITestResult to TestResult

[stevenaw]

Relates to #3423

There was an implicit cast in TestResult.AddToXml that could cause exceptions to be thrown when used with custom-derived implementations of ITestResult. This is a short-term fix to avoid the crash, but doesn't address the main point from @CharliePoole here: #3423 (comment)

[mikkelbu]

LGTM. Personally, I also think we should remove the same unnecessary casts in the test projects (both explicit casts and implicit casts in connection with `foreach), just to minimise the confusion. What do @nunit/framework-team think?

EDIT: And we also need an additional review 😄.

[ChrisMaddock]

Personally, I also think we should remove the same unnecessary casts in the test projects (both explicit casts and implicit casts in connection with `foreach), just to minimise the confusion. What do @nunit/framework-team think?

Absolutely, in my opinion. (Although I'm no longer framework team!)

Thanks for sorting this, @stevenaw!