[browser][tests] Activate System.Runtime.Extensions UserName tests (d…

…otnet#39692)

- Added specific tests for Browser

src/libraries/System.Runtime.Extensions/tests/System/Environment.UserName.cs

@@ -5,7 +5,6 @@
 
 namespace System.Tests
 {
-    [ActiveIssue("https://github.com/dotnet/runtime/issues/38164", TestPlatforms.Browser)]
     public class EnvironmentUserName
     {
         [Fact]
@@ -36,5 +35,14 @@ public void UserName_MatchesEnvironment_Windows()
         {
             Assert.Equal(Environment.GetEnvironmentVariable("USERNAME"), Environment.UserName);
         }
+
+        [Fact]
+        [PlatformSpecific(TestPlatforms.Browser)]
+        public void UserName_MatchesEnvironment_Browser()
+        {
+            string name = Environment.UserName;
+            Assert.False(string.IsNullOrWhiteSpace(name));
+            Assert.Equal("Browser", name);
+        }
     }
 }