[tests] Add --no-restore to dotnet invocations for better performance.

[jpobst]

Calling dotnet new <template> and dotnet add nuget <package> implicitly does a NuGet restore before returning.

However, after we get the test set up using these commands we call dotnet build which also does a NuGet restore.

Improve performance by opting out of these initial restores and only doing the one at build time.