Test project templates use latest C# language idioms

[DamianEdwards]

In .NET 6, the included C# project templates will use the latest language idioms.

See the parent Epic for details.

@kendrahavens

[Sanan07]

@DamianEdwards Should we add all mentioned features to our template? For instance, we do not have Program.cs file in our test template.
I assume only some of them applied in our case.

[Haplois]

@Sanan07, we cannot apply bellow yet:

• Top-level statements: The Test Platform has a bug which prevents tests declared in a class without a namespace to be run. [See: #test without namespace causes null ref microsoft/testfx#889]
• async Main: We generally don't have an entry point in our tests.

These can be considered:

• Global using directives Added nullable, and global namespaces. #167
• Nullable reference types Added nullable, and global namespaces. #167
• File-scoped namespaces Update templates to use file-scoped namespaces. #168

// cc: @nohwnd

[DamianEdwards]

Thanks @Haplois. Yes the intent is to make only those changes that are applicable and to follow up on any issues that are preventing a change that could be made, being made.

[Haplois]

File-scoped namespaces are blocked by dotnet/roslyn#54781.

[DamianEdwards]

@Haplois this should be unblocked now as of the latest SDK builds for preview.7 from main

[Haplois]

@DamianEdwards, thanks.