Add `dune-ctest`, a wrapper around `ctest`

CTest returns with an error status not only when tests failed, but also when tests were only skipped. This wrapper checks the log and returns successfully if no tests failed; skipped tests do not result in an error. This behaviour is needed in a continuous integration environment, when building binary packages or in other cases where the testsuite should be run automatically.

Closes issue #12 (closed)