merge: improve list tests

 * test: ignore mutt_exit()
 * test: add shared list testing code
 * test: improve test_mutt_list_clear()
 * test: improve test_mutt_list_compare()
 * test: improve test_mutt_list_find()
 * test: improve test_mutt_list_free()
 * test: improve test_mutt_list_free_type()
 * test: improve test_mutt_list_insert_after()
 * test: improve test_mutt_list_insert_head()
 * test: improve test_mutt_list_insert_tail()
 * test: improve test_mutt_list_match()