allow running tests in parallel

Currently the main makefile has only three real targets that iterate over all test folders via shell. This works fine, but prevents parallel testing, which gets more important the more / longer the tests take.

With the suggested change (copied from Stackoverflow) this works for every test directory added, as long as:

• regression from before: every directory in the test folder must contain tests (the wildcards could be adjusted to only look for "tests", but I didn't want to change the logic)
• each target used must be supported in every test directory (so far this was true for the three default ones only)

To run all tests in parallel one can now simply do make test -j (will create as much processes as test directories exist.