This removes three keys from the integration test matrix in favour of adding steps for each of those options to the job.

* The GitHub workspace is cleaned up between each run.
* The Composer cache is removed between each run.

This means that, in effect, the tests are being run in the same way as previously (clean install), but sequentially in one job instead of in parallel in different builds.

This slims down the number of builds per run from 197 to 29.

This adds the `tests/fixtures/no-lock-file` and `tests/fixtures/out-of-sync-lock` working directories to the matrix for the integration tests to test these situations more thoroughly on each commit as well.

Fixes 219

Add a job which tests "unclean" installs, i.e. running the action when there is already a `vendor` directory and a Composer `downloads` directory in place.

.. in the root `composer.json`.

... to allow for installing a wider range of versions of the `ehime/hello-world` package.

Includes locking the version for the `with-lock-file` test to `1.0.3` so there are higher and lower versions to switch to during the tests and switching the "unclean" test to use that fixture as a base.