test-run-tests.t: test running a failing test

tests/test-run-tests.t

@@ -18,3 +18,27 b' a succesful test'
   $ $TESTDIR/run-tests.py --with-hg=`which hg`
   .
   # Ran 1 tests, 0 skipped, 0 warned, 0 failed.
+
+failing test
+==================
+
+  $ cat > test-failure.t << EOF
+  >   $ echo babar
+  >   rataxes
+  > EOF
+
+  $ $TESTDIR/run-tests.py --with-hg=`which hg`
+  
+  --- $TESTTMP/test-failure.t
+  +++ $TESTTMP/test-failure.t.err
+  @@ -1,2 +1,2 @@
+     $ echo babar
+  -  rataxes
+  +  babar
+  
+  ERROR: test-failure.t output changed
+  !.
+  Failed test-failure.t: output changed
+  # Ran 2 tests, 0 skipped, 0 warned, 1 failed.
+  python hash seed: * (glob)
+  [1]