…mize validation tests.

- Add LowercaseLatinLetterPasswordValidator to check for minimum number of lowercase letters
- Replace LatinLetterValidator with ASCIIOnlyPasswordValidator
- Optimize validation tests using parameterized ones and add some new ones

- Introduce BaseAuthTestCase to handle common setup/teardown logic:
  - Predefine authentication-related URLs (signup, signin, protected, refresh)
  - Clean up User, BlacklistedToken and OutstandingToken records after tests
- Remove redundant setUp()/tearDown() methods in individual test classes
- Replace hardcoded reverse() calls with inherited URL attributes
- Improve test maintainability through centralized configuration

This refactoring follows DRY principles and makes test code more focused on actual test scenarios rather than boilerplate setup.