Allow cabal freeze tests to run on Windows

The tests now read cabal.config strictly so that the file is closed by the time
it is deleted for the next test.

cabal-install/tests/PackageTests/Freeze/Check.hs

@@ -110,4 +110,7 @@ removeCabalConfig = do
 
 readCabalConfig :: IO String
 readCabalConfig = do
-    readFile $ dir </> "cabal.config"
+    config <- readFile $ dir </> "cabal.config"
+    -- Ensure that the file is closed so that it can be
+    -- deleted by the next test on Windows.
+    length config `seq` return config