selftest: Move fork cmd args up a level

selftest: Move fork cmd args up a level

This is a fairly simple move of code and is the first step in a larger
refactor.

It doesn't matter if we build up the command args prior to the fork (we
only use them in the forked child). But moving the code means the code
to handle the fork-and-exec becomes common code that is repeated in
several places throughout Samba3.pm and Samba4.pm.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>