Add make memcheck tip for the HELP.md file.

[glennj]

I recently finished linked-list, my first exercise with malloc and free, and I was looking for instructions about how to check for leaked memory. I ended up on the track docs testing page which is kind of out of the way.

Are maintainers open to a PR for adding some words about make memcheck to exercises/shared/.docs/tests.md (so that it gets included in the HELP.md file that is downloaded)?

Perhaps something like

To check for memory leaks (allocated memory that is not freed), run:

```console
$ make memcheck
```

[ryanplusplus]

I think it's a good idea