Run yang validation in unit test

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#### What I did
Add unit test for db_migrator.py: ConfigDB passing yang validation.
Microsoft ADO: 24657445
Pending on sonic-net/sonic-buildimage#16974

#### How I did it
Add yang validation for unit test, and fix test data to pass yang validation.

#### How to verify it
Run sonic-utilities end to end test.

#### Previous command output (if the output of a command-line utility has changed)

#### New command output (if the output of a command-line utility has changed)

Author: ganglyu

scripts/db_migrator.py

@@ -1162,7 +1162,7 @@ def migrate(self):
             yang_parser.validate_data_tree()
         except sonic_yang.SonicYangException as e:
             syslog.syslog(syslog.LOG_CRIT, "Yang validation failed: " + str(e))
-            if os.environ["UTILITIES_UNIT_TESTING"] == "2":
+            if os.environ.get("UTILITIES_UNIT_TESTING", "0") == "2":
                 raise