unittests: use subfolders for different tests

Unittests for e.g. different libraries can be put into
`tests/unittests/tests-XXX`, where `XXX` is your test suite.

If `unittests` is made with `make all` (or any explicit argument), then
all test suites get built. If you use `make tests-XXX tests-YYY …` then
only the test suites `XXX` and `YYY` get built.