Fix PerformanceCounter tests

[danmoseley]

Fix #45182
Fix #54943

This has failed sporadically since these tests were written 4 years ago. On investigating I found that counter names and category names were intermingled, sometimes with the opposite names.
Changed to use consistent variable names and actual names. Added asserts. Moved deleting categories to after disposing the counter. Add retries on deleting category. Fix a few comments.

Fix #29753

Change to use a counter less likely to be zero, and retry it.

[ghost]

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Issue Details

---

Fix #45182
Fix #54943

This has failed sporadically since these tests were written 4 years ago. On investigating I found that counter names and category names were intermingled, sometimes with the opposite names.
Changed to use consistent variable names and actual names. Added asserts. Moved deleting categories to after disposing the counter. Add retries on deleting category. Fix a few comments.

Author:	danmoseley
Assignees:	-
Labels:	area-System.Diagnostics.Tracing
Milestone:	-

[danmoseley]

@steveisok Build tvOSSimulator x64 Release AllSubsets_Mono timed out. It says after 3 hours, but I think it's because the build took 80 minutes.