Various fixes to the CSharp setup (#15782)
### Description
<!-- Describe your changes. -->
Various fixes to the CSharp setup
- fix warnings
- fix invalid tests
- update test sdk nuget package
- enables testing on linux
- fixes issue with some unit tests not running in CI
- run unit tests in linux pipeline using dotnet
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
Unit tests weren't breaking in CIs for both Windows and Linux builds and
should have been.
