Add option to `extract_examples` to allow changing the directory the files are placed in.

[fingolfin]

This was suggested by @FriedrichRober : instead of putting the generated .tst files directly into .tst, it would be nice if one could also put them into a subdirectory; that way, one can separate generated files from manually handled files (which e.g. helps to prevent one from editing the generated test files manually).

One way to do this would be to add an entry subdir (or prefix or target_dir, or ...) to the extract_examples record. If present, then we change tst to something else... Some options:

1. subdir could be relative to the package directory, for maximal flexibility: so it would default to tst, but you could also use tst/generated or even some/other/path
2. subdir could be relative to the tst directory; so it would default to be empty, and we'd prefix it with tst/

It should be rather easy to implement this, too. The hardest part as usual is naming the option and deciding on its behavior ;-).