Add alert example that last at least 2h #39

This issue is part of #51 (Scenario library from real alerts), in the epic pyronear/roadmap#11 (End-to-end tests across engine, API and platform).

Find a real alert that lasted at least 2 hours, and add it as a scenario. It checks that a long fire stays one alert and does not split into several.

Done when

• Scenario added to the library
• Expected result written (one alert during the whole fire)

Theme Tests · Priority P2 · Period T4-26, T1-27