[fix:tests] Use config_app_label as class field in config selenium tests

[asmodehn]

Checklist

• I have read the OpenWISP Contributing Guidelines.
• I have manually tested the changes proposed in this pull request.
• I have written new test cases for new code and/or updated existing tests for changes to existing code.
• I have updated the documentation.

Description of Changes

Use a class attribute instead of hardcoding config_app_label in every url.
To ease extensions test maintainance.

[coderabbitai]

📝 Walkthrough

Walkthrough

This PR introduces a new class-level attribute config_app_label = "config" to the SeleniumTestMixin class in the test file. The attribute is used to dynamically construct admin URLs throughout the test methods. Hardcoded admin URL references (e.g., "admin:config_device_change") are replaced with dynamic equivalents that leverage the new attribute via reverse(f"admin:{self.config_app_label}_<entity>"). This refactoring affects multiple test navigation methods for device, devicegroup, VPN, and other entities.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• [change:tests] Added config_app_label in BaseTestNotification #1192 — Adds the same config_app_label class attribute to another test base class for dynamic admin URL construction with identical intent.

Suggested reviewers

• nemesifier

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description adequately explains the change's purpose (using class attribute instead of hardcoding to ease extension test maintenance) but is missing reference to existing issue and does not check all required checklist items.
Title check	✅ Passed	The title accurately describes the main change: introducing config_app_label as a class field in SeleniumTestMixin to replace hardcoded app label references in config selenium tests.

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[coveralls]

coverage: 98.641%. remained the same
when pulling c763979 on asmodehn:config_app_label_in_selenium_tests
into 6697a3a on openwisp:master.

[nemesifier]

Thanks @asmodehn