Fix PlatformDetection.IsThreadingSupported (#87863)

Author: pavelsavara

src/libraries/Common/tests/TestUtilities/System/PlatformDetection.cs

@@ -116,7 +116,8 @@ public static bool IsPrivilegedProcess
         public static bool FileCreateCaseSensitive => IsCaseSensitiveOS;
 #endif
 
-        public static bool IsThreadingSupported => !IsWasi && IsEnvironmentVariableTrue("IsBrowserThreadingSupported");
+        public static bool IsThreadingSupported => (!IsWasi && !IsBrowser) || IsWasmThreadingSupported;
+        public static bool IsWasmThreadingSupported => IsBrowser && IsEnvironmentVariableTrue("IsBrowserThreadingSupported");        
         public static bool IsBinaryFormatterSupported => IsNotMobile && !IsNativeAot;
 
         public static bool IsStartingProcessesSupported => !IsiOS && !IstvOS;