tests: transform test apps into unittests where it makes sense

This should fix issue #1131.

- [x] tests/bloom -> unittests/tests-bloom
- [ ] tests/bloom_bytes (unsure about this one; essentially a performance test)
- [ ] tests/net_if
- [ ] tests/posix_semaphore
- [x] tests/sha256 -> unittest/tests-crypto/tests-crypto-sha256.c (btw: currently, the test coverage for crypto is quite poor)
You can put that in as the second argument of `EMB_UNIT_TESTCALLER`