db: rework ongoing in TestCompactionPickerTargetLevel

In this test we specify ongoing compactions just by their levels, and the test code make some fairly arbitrary choices about which files are part of the compaction, e.g. for some reason Lbase compactions always include the entire base level (?)

We should switch to specifying the exact compactions, perhaps in a format similar to the one we use when logging compactions: `L1 [0001,0002] L2 [0003,00004] -> L5`

Jira issue: PEBBLE-357

Start with the TestCompactionPickerTargetLevel test and inspect how it currently specifies ongoing compactions by level. Compare that setup with the existing compaction logging format; done means the test specifies exact input files and target levels without relying on arbitrary file-selection behavior.

Written by the indexing model from the issue text.