Run .NET interop tests in parallel with all the other integration tests

[ForNeVeR]

Currently, we have a suite of integration tests (all run in parallel) and several .NET interop tests (C# + C), running sequentially.

We should somehow make them all run in parallel with each other, sharing the same parallelism pool.

Perhaps move the .NET interop tests into the common integration test assembly?

See Cesium.CodeGen.Tests.CodeGenNetInteropTests and TODO[#488] when implementing this.