test: fix conc038

We had spurious failures of conc038 test on CI with stdout:

```
 newThread started
-mainThread
-Haskell: 2
 newThread back again
+mainThread
 1 sec later

 shutting down
+Haskell: 2
```

testsuite/tests/concurrent/should_run/conc038.hs

@@ -24,9 +24,9 @@ main = do
   th <- newEmptyMVar
   forkIO $ do
      putStrLn "newThread started"
-     sleepBlock 1
+     sleepBlock 2
      putStrLn "newThread back again"
-     putMVar th "1 sec later"
+     putMVar th "2 secs later"
   threadDelay 500000 >> putStrLn "mainThread"
         -- this will not be blocked in the threaded RTS
   forkIO $ (hFun 2)

testsuite/tests/concurrent/should_run/conc038.stdout

@@ -2,6 +2,6 @@ newThread started
 mainThread
 Haskell: 2
 newThread back again
-1 sec later
+2 secs later
 
 shutting down