dtc: Assorted improvements to test harness

This patch makes several small improvements to the test harness.

* An altered way of invoking shell script testcases from run_tests.sh
  means scripts no longer need to me marked executable in the
  repository to work properly.

* dtc.sh never did anything that was really dtc specific - with the
  exception of messages, it would work equally well for any binary
  that returns 0 in the successful case.  Therefore, generalise dtc.sh
  and fold it into run_tests.sh so we don't need a separate script any
  more.

* Tweak various things so that the valgrind options are properly
  propagated down to invoke dtc under valgrind when called via wrapper
  scripts.

* Tweak the valgrind suppressions to work properly on a wider range of
  systems (this was necessary on my machine running Ubuntu Hardy).

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>