Fix test to take warning MSB5018 (#11499)

Fixes #9176

Context
Occasionally temp files fail to delete because of virus checkers, so generate MSB5018 warning.
The commit d7788d6#diff-3abd8382aac3bdfa59d5c1ca41dd089795d6ca539a00b3c50eab4fd6a0996314 including string lockedFileMessage = LockCheck.GetLockedFileMessage(fileName); doesn't output the related process.

Changes Made
Add the warning MSB5018

Author: JaynieBai

src/Tasks.UnitTests/Exec_Tests.cs

@@ -171,9 +171,20 @@ public void Timeout()
             Assert.Equal(expectedExitCode, exec.ExitCode);
             ((MockEngine)exec.BuildEngine).AssertLogContains("MSB5002");
             int warningsCount = ((MockEngine)exec.BuildEngine).Warnings;
-            warningsCount.ShouldBe(1,
+            if (warningsCount == 1)
+            {
+                warningsCount.ShouldBe(1,
                 $"Expected 1 warning, encountered {warningsCount}: " + string.Join(",",
                     ((MockEngine)exec.BuildEngine).WarningEvents.Select(w => w.Message)));
+            }
+            else
+            {
+                // Occasionally temp files fail to delete because of virus checkers, so generate MSB5018 warning
+                ((MockEngine)exec.BuildEngine).AssertLogContains("MSB5018");
+                warningsCount.ShouldBe(2,
+                $"Expected 2 warnings, encountered {warningsCount}: " + string.Join(",",
+                    ((MockEngine)exec.BuildEngine).WarningEvents.Select(w => w.Message)));
+            }
 
             // ToolTask does not log an error on timeout.
             Assert.Equal(0, ((MockEngine)exec.BuildEngine).Errors);