Create/Update a single task for each Apertis testcase

Due to the need to reboot the device multiple times, the aum testcases are composed of multiple test suites, one for each phase, under a single Apertis testcase.

Since the notification system relied on the suite name, it ended up creating multiple tasks for a single test failure.

By using the filename of the Apertis testcase as the key we ensure that a single task is create/updated for each testcase definition, as that's the granularity we care about.

By doing that, the code that searches for tasks in Phabricator can be simplified as well since we no longer have to distinguish between LAVA sub-testcases.