Change test display names in Azure Dev Ops

[vzarytovskii]

Right now, in Azure DevOps all the test categories look like "NUnit_TestResults_xxxxx_x", which is quite unreadable, and hard to see what's failing and where.
It needs to be fixed to have following structure (or close to it):

 Windows
 ├── netcoreapp3.1
 │      ├──  TestSuite1 (e.g. FSharp.Test.Compiler.ComponentTests)
 │      ├──  TestSuite2
 │      └──  ...          
 └── net472

 Linux
 └── netcoreapp3.1
        ├──  TestSuite1 (e.g. FSharp.Test.Compiler.ComponentTests)
        ├──  TestSuite2
        └──  ...          

 macOS
 └── netcoreapp3.1
        ├──  TestSuite1 (e.g. FSharp.Test.Compiler.ComponentTests)
        ├──  TestSuite2
        └──  ...          

This will allo wto easier pinpoint which test is failing on which platform.

[majocha]

This is more or less done.