test: fix up `make valgrind-test`

* valgrind complained too much about memory leaks from the V8 heap to be
  useful, run it with --leak-check=no. Not ideal, needs to be revisited,
  preferably with a suppression file.

* tools/run-valgrind.py didn't deal with tests that logged to stderr, rewrite
  the heuristic and make valgrind write to a socket instead of stderr.

Fixes #3869.