[tests] Enable TreatWarningsAsErrors=true

[pjcollins]

Fixes #16709
Sets TreatWarningsAsErrors to true when building template and sample
projects to ensure that warnings are not missed.

[pjcollins]

/azp run

[Eilon]

Yay! ❤️

It would be good to deliberately cause a warning (e.g. change a template/sample to call some obsolete API, or some wacky version) and ensure it causes the type of failure we want to see, and that the failure correctly causes the PR checks to fail. (And then of course revert the warning before merging 😁 )

[pjcollins]

I added a warning case to one of the templates for testing, but am not sure why CI isn't triggering...

[Eilon]

I added a warning case to one of the templates for testing, but am not sure why CI isn't triggering...

Maybe CI is just bogged down and needed a minute to recover. Looks like it's woken up now.

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[pjcollins]

I'll revert the testing changes, looks like this is working as expected:

[ToolRunner] Running: D:\a\_work\1\s\bin\dotnet\dotnet build "D:\a\_work\_temp\test-intg\Buildmauilibnet70ReleaseTrue\Buildmauilibnet70ReleaseTrue.csproj" -c Release -t:Pack -p:RestoreNoCache=true -p:RestorePackagesPath=D:\a\_work\_temp\test-intg\packages -p:RestoreConfigFile=D:\a\_work\_temp\test-intg\NuGet.config -p:CustomBeforeMicrosoftCSharpTargets=D:\a\_work\1\s\src\Templates\TemplateTestExtraTargets.targets -p:DisableTransitiveFrameworkReferenceDownloads=true -p:TreatWarningsAsErrors=true -bl:"D:\a\_work\_temp\test-intg\Buildmauilibnet70ReleaseTrue\Pack-133366053208885546.binlog"
[ToolRunner] Process 'dotnet' exited with code: 1
MSBuild version 17.5.1+f6fdcf537 for .NET
  Determining projects to restore...
  Restored D:\a\_work\_temp\test-intg\Buildmauilibnet70ReleaseTrue\Buildmauilibnet70ReleaseTrue.csproj (in 537 ms).
D:\a\_work\_temp\test-intg\Buildmauilibnet70ReleaseTrue\Class1.cs(6,9): error CS0169: The field 'Class1.unused' is never used [D:\a\_work\_temp\test-intg\Buildmauilibnet70ReleaseTrue\Buildmauilibnet70ReleaseTrue.csproj::TargetFramework=net7.0-ios]
D:\a\_work\_temp\test-intg\Buildmauilibnet70ReleaseTrue\Class1.cs(6,9): error CS0169: The field 'Class1.unused' is never used [D:\a\_work\_temp\test-intg\Buildmauilibnet70ReleaseTrue\Buildmauilibnet70ReleaseTrue.csproj::TargetFramework=net7.0-windows10.0.19041.0]
D:\a\_work\_temp\test-intg\Buildmauilibnet70ReleaseTrue\Class1.cs(6,9): error CS0169: The field 'Class1.unused' is never used [D:\a\_work\_temp\test-intg\Buildmauilibnet70ReleaseTrue\Buildmauilibnet70ReleaseTrue.csproj::TargetFramework=net7.0-maccatalyst]
D:\a\_work\_temp\test-intg\Buildmauilibnet70ReleaseTrue\Class1.cs(6,9): error CS0169: The field 'Class1.unused' is never used [D:\a\_work\_temp\test-intg\Buildmauilibnet70ReleaseTrue\Buildmauilibnet70ReleaseTrue.csproj::TargetFramework=net7.0-tizen]
D:\a\_work\_temp\test-intg\Buildmauilibnet70ReleaseTrue\Class1.cs(6,9): error CS0169: The field 'Class1.unused' is never used [D:\a\_work\_temp\test-intg\Buildmauilibnet70ReleaseTrue\Buildmauilibnet70ReleaseTrue.csproj::TargetFramework=net7.0-android]
Build FAILED.
D:\a\_work\_temp\test-intg\Buildmauilibnet70ReleaseTrue\Class1.cs(6,9): error CS0169: The field 'Class1.unused' is never used [D:\a\_work\_temp\test-intg\Buildmauilibnet70ReleaseTrue\Buildmauilibnet70ReleaseTrue.csproj::TargetFramework=net7.0-ios]
D:\a\_work\_temp\test-intg\Buildmauilibnet70ReleaseTrue\Class1.cs(6,9): error CS0169: The field 'Class1.unused' is never used [D:\a\_work\_temp\test-intg\Buildmauilibnet70ReleaseTrue\Buildmauilibnet70ReleaseTrue.csproj::TargetFramework=net7.0-windows10.0.19041.0]
D:\a\_work\_temp\test-intg\Buildmauilibnet70ReleaseTrue\Class1.cs(6,9): error CS0169: The field 'Class1.unused' is never used [D:\a\_work\_temp\test-intg\Buildmauilibnet70ReleaseTrue\Buildmauilibnet70ReleaseTrue.csproj::TargetFramework=net7.0-maccatalyst]
D:\a\_work\_temp\test-intg\Buildmauilibnet70ReleaseTrue\Class1.cs(6,9): error CS0169: The field 'Class1.unused' is never used [D:\a\_work\_temp\test-intg\Buildmauilibnet70ReleaseTrue\Buildmauilibnet70ReleaseTrue.csproj::TargetFramework=net7.0-tizen]
D:\a\_work\_temp\test-intg\Buildmauilibnet70ReleaseTrue\Class1.cs(6,9): error CS0169: The field 'Class1.unused' is never used [D:\a\_work\_temp\test-intg\Buildmauilibnet70ReleaseTrue\Buildmauilibnet70ReleaseTrue.csproj::TargetFramework=net7.0-android]
    0 Warning(s)
    5 Error(s)
Time Elapsed 00:00:07.18


  Failed Build("mauilib","net7.0","Release",True) [7 s]
  Error Message:
     Project Buildmauilibnet70ReleaseTrue.csproj failed to build. Check test output/attachments for errors.
  Expected: True
  But was:  False

  Stack Trace:
     at Microsoft.Maui.IntegrationTests.TemplateTests.Build(String id, String framework, String config, Boolean shouldPack) in D:\a\_work\1\s\src\TestUtils\src\Microsoft.Maui.IntegrationTests\TemplateTests.cs:line 45
   at InvokeStub_TemplateTests.Build(Object, Object, IntPtr*)

1)    at Microsoft.Maui.IntegrationTests.TemplateTests.Build(String id, String framework, String config, Boolean shouldPack) in D:\a\_work\1\s\src\TestUtils\src\Microsoft.Maui.IntegrationTests\TemplateTests.cs:line 45
   at InvokeStub_TemplateTests.Build(Object, Object, IntPtr*)

[Eilon]

Do you want to enable auto-merge so this can get merged once it passes CI?

[rmarinho]

Hum... not sure if this will block us of getting stuff merged .. but we will see.

[Eilon]

Hum... not sure if this will block us of getting stuff merged .. but we will see.

Yeah I think it's a worthwhile experiment. We'll see if it helps or hinders. But if it hinders a lot, then I think it points to a more serious problem of why we keep getting warnings showing up to begin with.