Assert that we don't get the value we don't want instead of attemptin…

…g to check for a specific error code. Fixes dotnet#39955 (dotnet#40019)

src/libraries/System.IO.FileSystem/tests/Enumeration/ErrorHandlingTests.cs

@@ -73,8 +73,11 @@ public void NotFoundErrorIsExpected()
             // Make sure we're returning the native error as expected (and not the PAL error on Unix)
             using (LastError le = new LastError(Path.GetRandomFileName()))
             {
-                // Conveniently ERROR_FILE_NOT_FOUND and ENOENT are both 0x2
-                Assert.Equal(2, le.Error);
+                // while ERROR_FILE_NOT_FOUND/ENOENT have predictable values on Windows, Linux and Mac,
+                //  we can't rely on ENOENT having the same value on other platforms. Instead, assert
+                //  that we didn't get the PAL error because we know its value.
+                const int PAL_Error_ENOENT = 0x1002D;
+                Assert.NotEqual(PAL_Error_ENOENT, le.Error);
             }
         }