osmo_fsm: Lookup functions to find FSM Instance by name or ID

Introduce two lookup helper functions to resolve a fsm_instance based on
the FSM and name or ID.  Also, add related test cases.

Change-Id: I707f3ed2795c28a924e64adc612d378c21baa815
diff --git a/include/osmocom/core/fsm.h b/include/osmocom/core/fsm.h
index 3a1f233..f42dd0c 100644
--- a/include/osmocom/core/fsm.h
+++ b/include/osmocom/core/fsm.h
@@ -141,6 +141,10 @@
 int osmo_fsm_register(struct osmo_fsm *fsm);
 void osmo_fsm_unregister(struct osmo_fsm *fsm);
 struct osmo_fsm *osmo_fsm_find_by_name(const char *name);
+struct osmo_fsm_inst *osmo_fsm_inst_find_by_name(const struct osmo_fsm *fsm,
+						 const char *name);
+struct osmo_fsm_inst *osmo_fsm_inst_find_by_id(const struct osmo_fsm *fsm,
+						const char *id);
 struct osmo_fsm_inst *osmo_fsm_inst_alloc(struct osmo_fsm *fsm, void *ctx, void *priv,
 					  int log_level, const char *id);
 struct osmo_fsm_inst *osmo_fsm_inst_alloc_child(struct osmo_fsm *fsm,