Rework testcase discovery

Unfortunately the lava section turned out too frail to rely on it, yielding entries missing critical data.

In particular, testcases with a deps: entry seem to be represented differently in the lava section, lacking the entry with all the metadata we are interested in.

This caused some testcases to not be detected at all, and their results not to be reported, like the apparmor-functional-demo one.

Rely on the job definition instead to discover the available testcases and rely on the lava section only to fetch some extra information such as the checked out commit id.

As a side-effect, the testcases no longer have a pre-defined URL on their own, so derive it from the URLs of the included tests. This avoids the case where a failing testcase links to a successful test, which caused confusion to users.