tests: remove LOCAL_SYSDB_FILE reference from test_sysdb_subdomains
Let's take advantage of the files provider and start to get rid of the
local provider references in our code.
Let's just replace "LOCAL_SYSDB_FILE" for NULL as This test suite
doesn't create local sysdb file.
Related:
https://pagure.io/SSSD/sssd/issue/3304
Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
Reviewed-by: Pavel Březina <pbrezina@redhat.com>