[db_migrator] Set correct CURRENT_VERSION, extend UT (sonic-net#2895)

Signed-off-by: vadymhlushko-mlnx <[email protected]>

What I did
Set the correct value for the db_migrator class variable CURRENT_VERSION, because if the new DB version was introduced the CURRENT_VERSION variable should have the newest version value.

How I did it
Edit the db_migrator.py.

How to verify it
Extend existing UT.

Author: vadymhlushko-mlnx

scripts/db_migrator.py

@@ -47,7 +47,7 @@ def __init__(self, namespace, socket=None):
                      none-zero values.
               build: sequentially increase within a minor version domain.
         """
-        self.CURRENT_VERSION = 'version_4_0_2'
+        self.CURRENT_VERSION = 'version_4_0_3'
 
         self.TABLE_NAME      = 'VERSIONS'
         self.TABLE_KEY       = 'DATABASE'

tests/db_migrator_test.py

@@ -674,4 +674,5 @@ def test_fast_reboot_upgrade_to_4_0_3(self):
         dbmgtr = db_migrator.DBMigrator(None)
         dbmgtr.migrate()
         expected_db = self.mock_dedicated_config_db(db_after_migrate)
-        assert not self.check_config_db(dbmgtr.configDB, expected_db.cfgdb)
+        assert not self.check_config_db(dbmgtr.configDB, expected_db.cfgdb)
+        assert dbmgtr.CURRENT_VERSION == expected_db.cfgdb.get_entry('VERSIONS', 'DATABASE')['VERSION']