tests: avoid calling test script to get target names

tests: avoid calling test script to get target names

As otherwise we couple *all* Makefile targets to the dependencies of
the test script, even for a simple make call (e.g., done on building
the source), so use a much simpler heuristic that just depends on
perl, which is essential in Debian.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>