Build is unreliable

[Galad]

Since the addition of .net core targets for test projects, the switch to dotnet test and the new VS 2019 image of AppVeyor, the build became unreliable. Issues are:

• Timeouts when installing chocolatey packages (jre8 mostly)
• Opencover does not produce the coverage sometimes (quality gate fails as a result)
• Some tests on timeouts fails randomly
• Error when building the solution, such as

C:\projects\tranquire\Directory.Build.targets(3,3): error : One or more errors occurred. (The process cannot access the file 'C:\Users\appveyor\AppData\Local\Temp\ab6210b7-e580-4ab0-b252-76c368c7f7fa.nuget.dgspec.json' because it is being used by another p