selftest: Add common function to return cmd environment variables

selftest: Add common function to return cmd environment variables

We prefix samba-tool commands with a bunch of WRAPPER/CONF environment
variables, in order for the command to work properly. These variables
are duplicated all over the place in the selftest code. This patch adds
a helper function to return the variables, so we can reduce the required
code down to a single line in a lot of places.

A couple of exceptions I've left alone:
- drs replicate, which omits the RESOLV_WRAPPER_CONF/_HOSTS variables
  (I'm not sure whether that's deliberate or not).
- create_backup(), which uses the backupfromdc's krb5.conf rather than
  the new testenv.

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