Use composer update instead of install

This will implicitly pull in dev dependencies and ensure that composer doesn't complain if composer.json and composer.lock (currently in gitignore) are out of sync.
Use CRUD's PHPUnit config when running tests

We still want to specify process isolation for the extension, but other options can rely on CRUD's own configuration.
Require composer deps before test tasks

Since we can't specify a dep on "test" itself, we'll require composer as the first dep of "testall". This should ensure that the fixture tests aren't skipped.