Let test target depend on the vast-test executable
Let test target depend on the vast-test executable

Due to a quirk in CTest, the `test` target is not a real target,
and can not be used with `add_dependencies()` the workaround is
to add a dedicated test that builds `vast-test` acts as a custom
dependency for all other tests.

vast-io/vast,vast-io/vast,vast-io/vast,vast-io/vast,vast-io/vast