Correct null device file name on Windows

Author: berberman

hls-test-utils/src/Test/Hls.hs

@@ -80,7 +80,7 @@ silenceStderr action =
     h `hDuplicateTo'` stderr
     bracket_ action (hClose old) (old `hDuplicateTo'` stderr)
   where
-    nullFile = if hostOS == Windows then "NUL" else "/dev/null"
+    nullFile = if hostOS == Windows then "\\\\.\\NUL" else "/dev/null"
 
 -- | Host a server, and run a test session on it
 -- Note: cwd will be shifted into @root@ in @Session a@