Extend RunTests target condition to also check that there are no tests to run

[carlossanlop]

In the dotnet/maintenance-packages repo, the CI is failing unexpectedly attempting to execute .NET Framework unit tests in Unix, even though we properly set the SkipTests property to true. This is happening when using dotnet build -t:test. The error output looks like this:

/home/carlos/.nuget/packages/microsoft.dotnet.arcade.sdk/8.0.0-beta.24165.4/tools/VSTest.targets(46,5): error MSB3073: The command "echo === COMMAND LINE === >  [/home/carlos/maintenance-packages/src/System.Runtime.CompilerServices.Unsafe/tests/System.Runtime.CompilerServices.Unsafe.Tests.csproj]
/home/carlos/.nuget/packages/microsoft.dotnet.arcade.sdk/8.0.0-beta.24165.4/tools/VSTest.targets(46,5): error MSB3073:                    echo "/home/carlos/maintenance-packages/.dotnet/dotnet" test  --logger:"console;verbosity=normal" --logger:"trx;LogFileName=" --logger:"html;LogFileName=" "--ResultsDirectory:" "--Framework:,Version=" >> "" 2>&1 >> " exited with code 2. [/home/carlos/maintenance-packages/src/System.Runtime.CompilerServices.Unsafe/tests/System.Runtime.CompilerServices.Unsafe.Tests.csproj]

The unexpected error output comes from inside the RunTests targets, which depends on the TestToRun items that are being used in the Outputs attribute. They come all the way from the _InnerGetTestsToRun target which does not get evaluated because it also depends on SkipTests not being true.

By adding the extra condition to the RunTests target to execute when SkipTests is not true and the TestToRun item group is not empty, we prevent that error.

While this gets fixed, we will be using a workaround in Directory.Build.targets in maintenance-packages to achieve a similar result:

  <Target Name="_SetSkipTests" AfterTargets="_OuterGetTestsToRun">

    <PropertyGroup>
      <SkipTests Condition="'@(TestToRun)' == ''">true</SkipTests>
    </PropertyGroup>

  </Target>

[ericstj]

/backport to release/8.0

[github-actions]

Started backporting to release/8.0: https://github.com/dotnet/arcade/actions/runs/8482496630