tests: don't use "status" operand of dd in test-censor.t (issue6858)

Some implementations don't have this operand, let's just direct stderr into
/dev/null, that's pretty cross-platform.

Also specify bs=512 (the default for me), because the default might be
different on different systems. Other uses of dd in the tests do specify it, so
this is more consistent.

tests/test-censor.t

@@ -300,7 +300,7 b' Can censor enough revision to move back '
   rev-count   data-size inl type      target 
           8         ??? no  file      target (glob) (revlogv2 !)
           8         ??? yes file      target (glob) (revlogv1 !)
-  $ cat /dev/rand?m | dd status=none count=200 | f --hexdump > target
+  $ cat /dev/rand?m | dd bs=512 count=200 2> /dev/null | f --hexdump > target
   $ hg ci -m 'add 100k passwords'
   $ H2=`hg id --debug -i`
   $ C5=$H2