tests: allow running migration tests in parallel

tests: allow running migration tests in parallel

It's not easily possible to use separate JSON files for the test configuration,
because part of it is generated with perl code. While this could be encoded too,
it seems cleaner to use the "run a single test by specifing the name"
functionality while adding a way for make to obtain a list of the test names.

Each test has (and needs) its own directory now, meaning the log files do not
need to be renamed anymore.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>