Change how we resolve dotnet in tests (#16934)

Author: BrennanConroy

src/Tools/dotnet-watch/test/Scenario/WatchableApp.cs

@@ -5,6 +5,7 @@
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
+using System.Reflection;
 using System.Runtime.CompilerServices;
 using System.Threading.Tasks;
 using Microsoft.Extensions.CommandLineUtils;
@@ -88,15 +89,19 @@ public void Start(IEnumerable<string> arguments, [CallerMemberName] string name
             };
             args.AddRange(arguments);
 
+            var dotnetPath = typeof(WatchableApp).Assembly.GetCustomAttributes<AssemblyMetadataAttribute>()
+                    .Single(s => s.Key == "DotnetPath").Value;
+            
             var spec = new ProcessSpec
             {
-                Executable = DotNetMuxer.MuxerPathOrDefault(),
+                Executable = dotnetPath,
                 Arguments = args,
                 WorkingDirectory = SourceDirectory,
                 EnvironmentVariables =
                 {
                     ["DOTNET_CLI_CONTEXT_VERBOSE"] = bool.TrueString,
                     ["DOTNET_USE_POLLING_FILE_WATCHER"] = UsePollingWatcher.ToString(),
+                    ["DOTNET_ROOT"] = Directory.GetParent(dotnetPath).FullName,
                 },
             };
 

src/Tools/dotnet-watch/test/dotnet-watch.Tests.csproj

@@ -17,6 +17,13 @@
     <ProjectReference Include="..\src\dotnet-watch.csproj" />
   </ItemGroup>
 
+  <ItemGroup>
+    <AssemblyAttribute Include="System.Reflection.AssemblyMetadataAttribute">
+      <_Parameter1>DotnetPath</_Parameter1>
+      <_Parameter2>$(DotNetTool)</_Parameter2>
+    </AssemblyAttribute>
+  </ItemGroup>
+
   <Target Name="CleanTestProjects" BeforeTargets="CoreCompile">
     <RemoveDir Directories="$(TargetDir)TestProjects" Condition="Exists('$(TargetDir)TestProjects')" />
   </Target>