Fix FSharp test race condition

Author: tmat

test/dotnet-watch.Tests/HotReload/ApplyDeltaTests.cs

@@ -80,7 +80,7 @@ public async Task ChangeFileInFSharpProject()
 
             UpdateSourceFile(Path.Combine(testAsset.Path, "Program.fs"), content => content.Replace("Hello World!", "<Updated>"));
 
-            await App.AssertOutputLineStartsWith("<Updated>");
+            await App.WaitUntilOutputContains("<Updated>");
         }
 
         [Fact]

test/dotnet-watch.Tests/Watch/Utilities/WatchableApp.cs

@@ -37,6 +37,14 @@ public WatchableApp(ITestOutputHelper logger)
         public static string GetLinePrefix(MessageDescriptor descriptor)
             => $"dotnet watch {descriptor.Emoji} {descriptor.Format}";
 
+        public async ValueTask WaitUntilOutputContains(string message)
+        {
+            if (!Process.Output.Any(line => line.Contains(message)))
+            {
+                _ = await AssertOutputLine(line => line.Contains(message));
+            }
+        }
+
         public Task<string> AssertOutputLineStartsWith(MessageDescriptor descriptor, Predicate<string> failure = null)
             => AssertOutputLineStartsWith(GetLinePrefix(descriptor), failure);