testsuite: Don't attempt to read from inherited fd in processT251

It turns out that this test is subtly broken. In particular, the
test will fail if any file is opened in the subprocess before the child
is run since the closed fd 0 may be reused for the new file.
This tends to happen in the threaded RTS due to the event manager's
control pipe (see GHC #22395). Unfortunately, it's not really clear how
else haskell#251 can reliably be tested.

tests/processT251.hs

@@ -30,7 +30,10 @@ child = do
 child2 :: IO ()
 child2 = do
     putStrLn "child2 start"
-    Left (IOError {ioe_type=InvalidArgument}) <-
-        try $ getContents >>= print
+    -- Unfortunate, there isn't a reliable way to test that stdin has been closed.
+    -- Afterall, if any file is opened in the child, it may reuse the
+    -- supposedly-closed fd 0. In particular this tends to happen in the
+    -- threaded RTS, since the event manager's control pipe is opened during
+    -- RTS initialzation.
     putStrLn "child2 done"