[tests] Fix import-time SQLite creation and add tmp_path fixtures for test isolation

[lin121291]

Changes:

• New tests/conftest.py - Centralized pytest fixtures for database management
• Modified tests/test_continuous_check.py - Refactored to use fixtures (-209 lines)
• Modified tests/test_route_postprocessing.py - Refactored to use fixtures (-64 lines)

Further Details:

Summary:

Problem:
The original test files created SQLite database files in the class body, which executes at import time rather than test runtime. This caused several issues:

1. File pollution: Database files created in working directory without cleanup
2. Naming conflicts: Both test files used the same filename gdfDB.sqlite
3. Poor test isolation: Global engine variables shared across tests
4. Anti-pattern: I/O operations executing at import time in class body

Solution:
Replace ad-hoc database creation with proper pytest fixtures:

• Database files now created in temporary directories via tmp_path_factory
• Automatic cleanup after test completion
• Proper dependency injection instead of global variables
• Improved test isolation with separate databases per test class

Behaviour before: Tests created gdfDB.sqlite in working directory at import time, file remained after tests
Behaviour after: Tests create databases in temporary directories during test execution, automatic cleanup

Dependencies:

No new dependencies added.

PR Checklist:

In the context of this PR, I:

• have filled the 52North Contributor License Agreement and am waiting for feedback
• provide unit tests embedded in the WRT test framework (WeatherRoutingTool/tests) that allow the simple testing of the new/modified functionalities. All (previous and new) unit tests execute without new error messages.
• ensure that the code formatter runs without errors/warnings
• ensure that my changes follow the WRT's guidelines for contributing at the time of the contribution

[kdemmich]

Hi @lin121291, thank you for your PR. We appreciate the time and effort that was put into it. Please be aware that we require to follow 52 North's CLA guidelines for all PRs and, in particular, fill the 52°North Contributor License Agreement. As long as this has not been filled, I can not assign you to any issues or merge any PRs. Please don't hesitate to contact me in case of related questions.