Outer/inner setup : allow append to outer args, use a bigger --num-callers

Outer/inner setup : allow append to outer args, use a bigger --num-callers
* Similarly to what was done in revision r16104 for vg_perf, have a way
  to append to predefined vg_regtest outer args, rather than only be able
  to replace them.
* increase the num callers for outer tool to 40.
  (This is in preparation of a future commit, that will allow outer tool
  to append the inner guest stack trace to reported errors in the inner)

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@16133