Reduce AtomicModifyIORef increment count

This test leads to a lot of contention when N>2 and becomes very slow. Let's reduce the amount of work we do to compensate. Resolves #24490

libraries/base/tests/AtomicModifyIORef.hs

@@ -5,7 +5,7 @@ import Data.IORef
 main :: IO ()
 main = do
     let nThreads = 10
-        nIncrs = 10000000
+        nIncrs = 10000
 
     ref <- newIORef (42 :: Int)
     dones <- replicateM nThreads $ do

libraries/base/tests/AtomicModifyIORef.stdout

@@ -8,4 +8,4 @@
 .
 .
 .
-100000042
+100042