Narrow test filtering to single-threaded

Author: kg

src/libraries/System.Console/tests/SyncTextWriter.cs

@@ -12,8 +12,8 @@
 
 public class SyncTextWriter
 {
-    [Fact]
-    [SkipOnPlatform(TestPlatforms.Browser, "Browser bypasses SyncTextWriter for faster startup")]
+    // Browser bypasses SyncTextWriter for faster startup
+    [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsThreadingSupported))]
     public void SyncTextWriterLockedOnThis()
     {
         TextWriter oldWriter = Console.Out;

src/libraries/System.Runtime/tests/System.IO.Tests/StreamReader/StreamReader.cs

@@ -29,8 +29,8 @@ public void ObjectClosedReadLineBaseStream()
             Assert.Throws<ObjectDisposedException>(() => sr.ReadLine());
         }
 
-        [Fact]
-        [SkipOnPlatform(TestPlatforms.Browser, "Browser bypasses SyncTextWriter for faster startup")]
+        // Browser bypasses SyncTextWriter for faster startup
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsThreadingSupported))]
         public void Synchronized_NewObject()
         {
             using (Stream str = GetLargeStream())

src/libraries/System.Runtime/tests/System.IO.Tests/StreamWriter/StreamWriter.cs

@@ -7,8 +7,8 @@ namespace System.IO.Tests
 {
     public partial class WriteTests
     {
-        [Fact]
-        [SkipOnPlatform(TestPlatforms.Browser, "Browser bypasses SyncTextWriter for faster startup")]
+        // Browser bypasses SyncTextWriter for faster startup
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsThreadingSupported))]
         public void Synchronized_NewObject()
         {
             using (Stream str = CreateStream())

src/libraries/System.Runtime/tests/System.IO.Tests/TextWriter/TextWriterTests.cs

@@ -690,8 +690,8 @@ public void DisposeAsync_ExceptionReturnedInTask()
             Assert.Same(e, vt.AsTask().Exception.InnerException);
         }
 
-        [Fact]
-        [SkipOnPlatform(TestPlatforms.Browser, "Browser bypasses SyncTextWriter for faster startup")]
+        // Browser bypasses SyncTextWriter for faster startup
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsThreadingSupported))]
         public async Task FlushAsync_Precanceled()
         {
             Assert.Equal(TaskStatus.RanToCompletion, TextWriter.Null.FlushAsync(new CancellationToken(true)).Status);