[Y-Cable] Increased unit test coverage of y_cable_helper.py (#229)

Added unit tests for the following methods:

y_cable_wrapper_get_transceiver_info
y_cable_wrapper_get_presence
y_cable_toggle_mux_torA
y_cable_toggle_mux_torB
format_mapping_identifier
update_appdb_port_mux_cable_response_table
update_table_mux_status_for_response_tbl
update_tor_active_side
read_y_cable_and_update_statedb_port_tbl
delete_port_from_y_cable_table
create_tables_and_insert_mux_unknown_entries
check_identifier_presencead_update_mux_table_entry
init_ports_status_for_y_cable
change_ports_status_for_y_cable_change_event
delete_ports_status_for_y_cable
get_firmware_dict
get_muxcable_info
get_muxcable_static_info
post_mux_info_to_db
task_download_firmware_worker

Motivation and Context
The motivation for these changes stems from needing to increase the unit test coverage of the y_cable_helper methods to above 50%.

How Has This Been Tested?
This was tested by building and running the new unit tests and verifying that they contribute to and increase the code coverage as reflected in the xcvrd_xcvrd_utilities_y_cable_helper_py.html page.

Co-authored-by: Ashwin Srinivasan <ashwin.srinivasan@microsoft.com>

sonic-xcvrd/tests/mock_swsscommon.py

@@ -6,14 +6,17 @@ class Table:
     def __init__(self, db, table_name):
         self.table_name = table_name
         self.mock_dict = {}
+        self.mock_keys = []
 
     def _del(self, key):
         if key in self.mock_dict:
             del self.mock_dict[key]
+            self.mock_keys.remove(key)
         pass
 
     def set(self, key, fvs):
         self.mock_dict[key] = fvs
+        self.mock_keys.append(key)
         pass
 
     def get(self, key):
@@ -23,6 +26,9 @@ def get(self, key):
 
     def get_size(self):
         return (len(self.mock_dict))
+
+    def getKeys(self):
+        return self.mock_keys
 
 
 class FieldValuePairs: