Disable Process BasePriority test on WSL (#25869)

https://github.com/dotnet/corefx/issues/25861

Author: joshfree

src/System.Diagnostics.Process/tests/ProcessTests.Unix.cs

@@ -238,12 +238,20 @@ public void TestBasePriorityOnUnix()
             ProcessPriorityClass originalPriority = _process.PriorityClass;
             Assert.Equal(ProcessPriorityClass.Normal, originalPriority);
 
-            SetAndCheckBasePriority(ProcessPriorityClass.Idle, 19);
+            // https://github.com/dotnet/corefx/issues/25861 -- returns "-19" and not "19"
+            if (!PlatformDetection.IsWindowsSubsystemForLinux)
+            {
+                SetAndCheckBasePriority(ProcessPriorityClass.Idle, 19);
+            }
 
             try
             {
                 SetAndCheckBasePriority(ProcessPriorityClass.Normal, 0);
-                SetAndCheckBasePriority(ProcessPriorityClass.High, -11);
+                // https://github.com/dotnet/corefx/issues/25861 -- returns "11" and not "-11"
+                if (!PlatformDetection.IsWindowsSubsystemForLinux)
+                {
+                    SetAndCheckBasePriority(ProcessPriorityClass.High, -11);
+                }
                 _process.PriorityClass = originalPriority;
             }
             catch (Win32Exception ex)