Merge pull request #4449 from jtattermusch/csharp_test_restructuring

src/csharp/Grpc.Core.Tests/Grpc.Core.Tests.csproj

src/csharp/Grpc.Core.Tests/Internal/TimespecTest.cs

src/csharp/Grpc.Core.Tests/PInvokeTest.cs

src/csharp/Grpc.Core.Tests/PerformanceTest.cs

src/csharp/Grpc.Core.Tests/SanityTest.cs

...csharp/Grpc.IntegrationTesting/InteropClientServerTest.cs

src/csharp/Grpc.IntegrationTesting/RunnerClientServerTest.cs

src/csharp/tests.json

tools/run_tests/build_csharp.sh

tools/run_tests/run_csharp.bat

tools/run_tests/run_csharp.sh

tools/run_tests/run_tests.py