Disable testhost prestart (#4833) (#4836)

* Disable testhost prestart

* Fix test

Author: nohwnd

src/Microsoft.TestPlatform.CrossPlatEngine/Client/Parallel/ParallelOperationManager.cs

@@ -18,7 +18,7 @@ namespace Microsoft.VisualStudio.TestPlatform.CrossPlatEngine.Client;
 /// </summary>
 internal sealed class ParallelOperationManager<TManager, TEventHandler, TWorkload> : IDisposable
 {
-    private const int PreStart = 2;
+    private const int PreStart = 0;
     private readonly static int VSTEST_HOSTPRESTART_COUNT =
         int.TryParse(
                 Environment.GetEnvironmentVariable(nameof(VSTEST_HOSTPRESTART_COUNT)),

test/Microsoft.TestPlatform.CrossPlatEngine.UnitTests/Client/Parallel/ParallelProxyExecutionManagerTests.cs

@@ -285,11 +285,7 @@ public void StartTestRunShouldProcessAllSourcesOnExecutionAbortsForAnySource()
 
         Assert.IsTrue(_executionCompleted.Wait(Timeout3Seconds), "Test run not completed.");
 
-        // Even though we start the test run for two sources, because of the current setup where
-        // we initialize a proxy if no more slots are available, we end up with abort notice being
-        // sent only to the running manager. This leaves the initialized manager in limbo and the
-        // assert will fail because of this.
-        Assert.AreEqual(1, _processedSources.Count, "Abort should stop all sources execution.");
+        Assert.AreEqual(2, _processedSources.Count, "Abort should stop all sources execution.");
     }
 
     [TestMethod]