Add PublishReadyToRun tests

[tmds]

Implements #85

cc @omajid @RheaAyase

[tmds]

Both test fail (as expected)

+ dotnet publish -r fedora.30-x64 /p:PublishReadyToRun=true
Microsoft (R) Build Engine version 16.3.0-preview-19426-01+faf5e5d75 for .NET Core
Copyright (C) Microsoft Corporation. All rights reserved.

/home/tmds/repos/dotnet-bunny/dotnet-regular-tests/publish-ready-to-run/publish-ready-to-run.csproj : error NU1101: Unable to find package Microsoft.NETCore.App.Runtime.fedora.30-x64. No packages exist with this id in source(s): nuget.org

+ dotnet publish -r linux-x64 /p:PublishReadyToRun=true
Microsoft (R) Build Engine version 16.3.0-preview-19426-01+faf5e5d75 for .NET Core
Copyright (C) Microsoft Corporation. All rights reserved.

  Restore completed in 109.82 ms for /home/tmds/repos/dotnet-bunny/dotnet-regular-tests/publish-ready-to-run-linux/publish-ready-to-run-linux.csproj.
  You are using a preview version of .NET Core. See: https://aka.ms/dotnet-core-preview
  publish-ready-to-run-linux -> /home/tmds/repos/dotnet-bunny/dotnet-regular-tests/publish-ready-to-run-linux/bin/Debug/netcoreapp3.0/linux-x64/publish-ready-to-run-linux.dll
/usr/lib64/dotnet/sdk/3.0.100-preview9-014004/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Publish.targets(252,5): error NETSDK1095: Optimizing assemblies for performance is not supported for the selected target platform or architecture. Please verify you are using a supported runtime identifier, or set the PublishReadyToRun property to false. [/home/tmds/repos/dotnet-bunny/dotnet-regular-tests/publish-ready-to-run-linux/publish-ready-to-run-linux.csproj]

[omajid]

LGTM. Thanks!