What is better than fixing bugs? Fixing entire bug classes of course! :)

Add option `--valgrind` to run the functional tests under Valgrind.

Regular functional testing under Valgrind would have caught many of the uninitialized reads we've seen historically.

Let's kill this bug class once and for all: let's never use an uninitialized value ever again. Or at least not one that would be triggered by running the functional tests! :)

My hope is that this addition will make it super-easy to run the functional tests under Valgrind and thus increase the probability of people making use of it :)

Hopefully `test/functional/test_runner.py --valgrind` will become a natural part of the pre-release QA process.

**Usage:**

```
$ test/functional/test_runner.py --help
…
  --valgrind            run nodes under the valgrind memory error detector:
                        expect at least a ~10x slowdown, valgrind 3.14 or
                        later required
```

ninja
test_runner.py --valgrind