This condition is invalid because different_realm is not set, when
EINVAL is returned. It can make the test fail sometimes.

https://fedorahosted.org/sssd/ticket/1833

state is expected to be a pointer

There was shared code for several unit tests that connected to their own
sysdb instance. This patch adds common code to run a generic request to
completion or connect to a test sysdb.

This prevents reportin false errors when internal functions return
a generic EINVAL or EACCES that should just be treated as internal
errors.

- Use a double-linked list for domains and subdomains.
- Never remove a subdomain, simply mark it as disabled if it becomes
  unused.
- Rework the way subdomains are refreshed.
  Now sysdb_update_subdomains() actually updates the current subdomains
  and marks as disabled the ones not found in the sysdb or add new ones
  found. It never removes them.
  Removal of missing domains from sysdb is deferred to the providers,
  which will perform it at refresh time, for the ipa provider that is
  done by ipa_subdomains_write_mappings() now.
  sysdb_update_subdomains() is then used to update the memory hierarchy
  of the subdomains.

- Removes sysdb_get_subdomains()
- Removes copy_subdomain()
- Add sysdb_subdomain_delete()

struct sss_domain_info is always used to represent domains now.
Adjust tests accordingly.