[workload] add support for 'dotnet test' and 'net6.0'

[jonathanpeppers]

Description of Change

Fixes: dotnet/sdk#21845

If you create a net6.0 class library with UseMaui=true, dotnet test fails with:

Testhost process exited with error: It was not possible to find any compatible framework version
The framework 'Microsoft.Maui.Core', version '**FromWorkload**' (x64) was not found.

@dsplaisted suggested adding %(RuntimePackAlwaysCopyLocal) to any
@(KnownFrameworkReference) that is meant for net6.0:

dotnet/sdk#21845 (comment)

If a class library does not have a platform specified, then the
Microsoft.Maui.*.any packs are brought in. These packs need
%(RuntimePackAlwaysCopyLocal) for dotnet test to work.

I added a sample project that appears to work now:

> .\bin\dotnet\dotnet.exe test .\src\Controls\samples\Controls.Sample.Tests\Controls.Sample.Tests.csproj
Determining projects to restore...
Restored C:\src\maui\src\Controls\samples\Controls.Sample.Tests\Controls.Sample.Tests.csproj (in 1.89 sec).
You are using a preview version of .NET. See: https://aka.ms/dotnet-core-preview
Controls.Sample.Tests -> C:\src\maui\src\Controls\samples\Controls.Sample.Tests\bin\Debug\net6.0\Controls.Sample.Tests.dll
Test run for C:\src\maui\src\Controls\samples\Controls.Sample.Tests\bin\Debug\net6.0\Controls.Sample.Tests.dll (.NETCoreApp,Version=v6.0)
Microsoft (R) Test Execution Command Line Tool Version 17.0.0
Copyright (c) Microsoft Corporation.  All rights reserved.
Starting test execution, please wait...
A total of 1 test files matched the specified pattern.
Passed!  - Failed:     0, Passed:     1, Skipped:     0, Total:     1, Duration: < 1 ms - Controls.Sample.Tests.dll (net6.0)

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)
• Targets a single property for a single control (or intertwined few properties)
• Adds the property to the appropriate interface
• Avoids any changes not essential to the handler property
• Adds the mapping to the PropertyMapper in the handler
• Adds the mapping method to the Android, iOS, and Standard aspects of the handler
• Implements the actual property updates (usually in extension methods in the Platform section of Core)
• Tags ported renderer methods with [PortHandler]
• Adds an example of the property to the sample project (MainPage)
• Adds the property to the stub class
• Implements basic property tests in DeviceTests

Does this PR touch anything that might affect accessibility?

No

[jonathanpeppers]

I see my new test, going to merge this.