tests: Improve asserts

This MR proposes a new set of assertions and helpers to ease the reading and writing of tests.

The test assertions are based on the new Weston assertion system. zunitc has been removed and the 2 tests using it have been ported to the more common Weston testing framework. Using custom asserts based on the Weston assertion system instead of relying on libc's assert() will also help providing NDEBUG builds. This is currently not possible because setting NDEBUG would disable all the asserts in the tests.