selftest: Change backup/restore testenvs to use 1 prefork child

selftest: Change backup/restore testenvs to use 1 prefork child

Recently the gitlab CI jobs were hitting memory resource limits and
using swap, which then caused test failures. The process model used in
the testenvs seemed to be contributing to this problem.

We can reduce the memory overhead of the restore/backup testenvs by
using 1 prefork child process instead of the default of 4 (kudos to
Garming for the idea). The tests run against these testenvs are basic
sanity-checks, rather than heavy-duty stress tests, so the number of
prefork workers shouldn't matter.

This is a bit of a tradeoff between testing the defaults that will
actually be used in production vs using limited resources efficiently on
shared CI runner machines.

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