selftest: Add common fork_and_exec() function

selftest: Add common fork_and_exec() function

Now the code has been refactored, we can move it into a common function.
This reduces code duplication and means we have a common place where we
start samba daemons from.

Note that some daemons behave slightly different, but the $daemon_ctx
allows us to customize their behaviour a bit.

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