IDMAP: Add test to validate off by one bug
Resolves:
https://fedorahosted.org/sssd/ticket/2922
Reviewed-by: Pavel Březina <pbrezina@redhat.com>
Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
Reviewed-by: Michal Židek <mzidek@redhat.com>
(cherry picked from commit 9d17f436795a36b1b1126f444923aa847fd0f93a)