Normalize_NonWindows_AddsLeadingForwardSlash was not previously running on merge PRs

[ryzngard]

With #11510 the test was moved and appears to be running on MacOS/Linux correctly where previously it appears that it wasn't. See https://dev.azure.com/dnceng-public/public/_build/results?buildId=993852&view=ms.vss-test-web.build-test-results-tab for a run where it is not run at all.

I'm not sure why it wasn't running, but that project is going away so the test should just be fixed.

[ryzngard]

Likely this is the cause:

razor/eng/Build.props

Line 14 in c598cfa

<ProjectToBuild Condition="'$(OS)'!='WINDOWS_NT' and '$(SdkTaskProjects)'==''" Include="$(MSBuildThisFileDirectory)..\Razor.Slim.slnf" />

ProjectEngineHost was not in the slnf file but the new location/test location are. This test would have failed prior