The answer is similar to option 1 in eng.augusto’s answer.
Microsoft doesn’t provide NuGet for the latest version of Microsoft.VisualStudio.QualityTools.UnitTestFramework,
but rather supplies it as a part of Visual Studio
(normally at C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\Common7\IDE\PublicAssemblies\Microsoft.VisualStudio.QualityTools.UnitTestFramework.dll)
I created the folder Microsoft.VisualStudio.QualityTools as a subfolder of my solution and copied:
Microsoft.VisualStudio.QualityTools.UnitTestFramework.dll
Microsoft.VisualStudio.QualityTools.UnitTestFramework.xml
The files should be added to source control (even if DLLs are usually ignored).
Then I changed references in my Test.csproj to refer to a new location.