Add a test for Data.Map, for a bug on the libraries@ list

libraries/base/tests/Makefile

+# This Makefile runs the tests using GHC's testsuite framework.  It
+# assumes the package is part of a GHC build tree with the testsuite
+# installed in ../../../testsuite.
+
+TOP=../../../testsuite
+include $(TOP)/mk/boilerplate.mk
+include $(TOP)/mk/test.mk

libraries/base/tests/all.T

+# This is a test script for use with GHC's testsuite framework, see
+# http://darcs.haskell.org/testsuite
+
+test('datamap001', normal, compile_and_run, [''])
+

libraries/base/tests/datamap001.hs

+
+{-
+In the 6.6 era this printed [(5,"x")]; should be [(3,"b"),(5,"a")]
+-}
+
+module Main (main) where
+
+import Data.Map
+
+main :: IO ()
+main = do let m = fromList [(3,"b"),(5,"a")]
+              f k a = Just "x"
+              m' = updateAt f 1 m
+          print m'

libraries/base/tests/datamap001.stdout

+fromList [(3,"b"),(5,"x")]