[test gc] add real gc stress tests #1108

stress test all GCs, under tight memory --gc-nursery-size=0.0001
and additional --gc-debug runs, with additonal StringBuilder stress via -t
and check the exit codes.
Properly fix SIGNAL detection and error output in Parrot::Test for exit_code_is.
Fails 14/20 tests.

t/harness: add existing TEST_PROG_ARGS to tests single tests, e.g.
  ulimit -Sv 60000
  TEST_PROG_ARGS='--gc-nursery-size=0.05 ' perl t/harness --gc-debug t/pmc/*.t