tests: Split out setUp code into separate function for reuse

tests: Split out setUp code into separate function for reuse

Any test that wants to change a password has to set the dSHeuristics
and minPwdAge first in order for the password change to work. The code
that does this is duplicated in several tests. This patch splits it out
into a static method so that the code can be reused rather than
duplicated.

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