net: conn_mgr: Minor fixes

- Add missing event sleep after taking all ifaces up in
  test_connect_disconnect.
- Add missing event sleep after resetting ifaces in conn_mgr_conn_before
- Fix typo in comment for internal state flags.
- Add missing NET_MGMT_EVENT_BIT to conn_mgr_connectivity event
  definitions.
- Missing net_mgmt.h include in conn_mgr_connectivity.h
- Split conn_mgr_conn iface reset into network and state resets, before
  and after event sleep, so that triggered events do not corrupt the
  state reset.
- Reduce SIMULATED_EVENT_DELAY to 100ms to avoid timeouts on real-time
  targets.
- Use macro for simulated event wait times.

Signed-off-by: Georges Oates_Larsen <georges.larsen@nordicsemi.no>