Allow removing output formatter from unittest

[chrisheller]

Summary

It is possible to add output formatters to unittest, but it is not possible to remove them.

Description

This is an issue for the JUnit output format (see #2088) when you have more than one test suite that are creating their own output files. Once the first suite is done and closes its output stream, the output formatter is still in the seq of formatters that is maintained inside of unittest.nim. Once the second test suite starts producing output, an exception is raised since the stream is closed.

Alternatives

Alternative implementations could include the ability to mark an OutputFormatter as no longer active, but that would require updating other code in unittest.nim to look for that flag and honor it. Being able to just remove the formatter when a suite is finished seems simpler.

[ghost]

Should've been closed by #12032 - doing that now :)