Use the GTest convention for naming unit and integration tests

Details can be found here. In particular, the contraindication on underscores in suite and test names shall be strictly followed.

Use the zstrong::tests namespace for unit and integration tests

All TEST, TEST_F, and similar classes should be declared within the zstrong::tests namespace or, rarely, a sub-namespace if appropriate. Helper functions and classes should be put into an inner anonymous namespace to avoid name collisions. The exception to this rule is utility objects shared between files, which should also be prefixed with the zstrong::tests namespace.

Directory organization (WIP)

Unit tests shall be placed in the tests/unittest directory.