maintcheck: fix spurious failure in 'maintainer-check-list-of-tests'

maintcheck: fix spurious failure in 'maintainer-check-list-of-tests'

* Makefile.am (test_subdirs): Add 'contrib/t', otherwise we would get a
spurious error like:

  List of tests in Makefile an on filesystem differ
  + diff -u in-makefile on-filesystem
  --- tests-in-makefile-list.tmp    2012-07-03 23:07:47.000000000 +0200
  +++ tests-on-filesystem-list.tmp  2012-07-03 23:07:47.000000000 +0200
  @@ -1,7 +1,3 @@
  -contrib/t/help-multilib.sh
  -contrib/t/multilib.sh
  -contrib/t/parallel-tests-html-recursive.sh
  -contrib/t/parallel-tests-html.sh
   t/ac-output-old.tap
   t/acloca10.sh
   t/acloca11.sh
  make: *** [maintainer-check-list-of-tests] Error 1

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>