Use assertCompiles in accessible and mockable tests

[ioleo]

Next release of ZIO introduces assertCompiles. We should refactor current AccessibleSpec and MockableSpec tests to leverage assertCompiles(...) and not(assertCompiles(...)).

Note: this issue requires bumping the dependency on zio-test to next version (not yet releleased). The current latest 1.0.0-RC16 does not contain assertCompiles yet.