Change test suite to not write into package directory / allow building manuals "out of tree"

[fingolfin]

... so that it is possible to run the tests from a read-only copy of the package (e.g. installed via a Linux package manager systemwide).

Basically tst/dogfood.tst does this:

# regenerate the manual using AutoDoc
gap> Read("makedoc.g");

It would be nice if one could specify an "output" directory when calling AutoDoc.

However I think this really needs changes in GAPDoc (or else a hack where we copy lots of files into the output dir).