Add runtests subcommand

[shoo]

As with many official tools, projects often require integration testing for CI/CD. In such cases, it is useful to run integration tests in a unified format. The runtests subcommand I added is similar to test/run-unittest.sh or test/run-unittest.d so integration tests of dub projects can be run dub itself. To test dub, run the following command:

dub run -- runtests --skip-build