[tests] Test Failures with Python 3.12: assertDictContainsSubset and assertEquals Deprecation

[dee077]

Summary:

Tests in OpenWISP Controller fail on Python 3.12 due to:

1. Removal of assertDictContainsSubset.
2. Deprecation of assertEquals (now replaced by assertEqual).

Steps to Reproduce:

1. Set up OpenWISP Controller and install dependencies.
2. Run tests with Python 3.12.
3. Two test failures:
   • assertDictContainsSubset in test_context_regression.
   • assertEquals in test_ip_fields_not_duplicated.

Expected Behavior:
Tests should run successfully on Python 3.12, with updated assertions.

Error Messages:

• assertDictContainsSubset: AttributeError: module unittest has no attribute assertDictContainsSubset
• assertEquals: DeprecationWarning: The assertEquals method is deprecated, use assertEqual instead.

Suggested Fix:

• Replace assertDictContainsSubset with assertDictEqual.
• Replace assertEquals with assertEqual.