Use composer for dependency resolution when running tests

DrupalCI should support using composer for dependency resolution when running tests - and in particularly allow the specification of a separate composer.json file for running tests against the dev branch of a project.

By allowing module developers to use their normal composer.json file for stable testing and regression testing in nightly and commit tests, but to also specify a different composer.json for testing the dev branch, developers will be able to test against dev versions of dependencies that may fix important upstream bugs.