.NET Framework test runs are _super_ slow in CI

(Put this under https://github.com/dotnet/fsharp/labels/Contributor%20Pain, I guess.)

When did the Windows desktop (.NET Framework) build+test runs start taking ~3× as long as the other build+test runs? Was this an expected outcome of some recent change?

The `Linux`, `MacOS`, `WindowsNoRealsig_testCoreclr`, etc., runs take about half an hour now, while the `WindowsNoRealsig_testDesktop` and `WindowsCompressedMetadata desktop_release` runs now take **more than an hour and a half**.

I feel like they didn't take that long even just a couple months ago, either absolutely or relatively. I remember them taking maybe 55 minutes or something (although Azure Pipelines runs older than 30 days have been purged, so I guess I can't prove it).