Cover `:shorten` option

[vemv]

https://github.com/clojure-emacs/enrich-classpath/pull/18 introduced the :shorten? option (which will become the default) but back then I could only add a few unit tests.

This PR adds more integration/e2e tests and misc reliability improvements I found along the way.

Closes https://github.com/clojure-emacs/enrich-classpath/issues/21

[vemv]

• Error: cannot resolve cider.enrich-classpath/middleware middleware
• error: error reading /home/circleci/.m2/repository/org/eclipse/jetty/websocket/websocket-servlet/9.4.51.v20230217/websocket-servlet-9.4.51.v20230217-javadoc.jar; zip file is empty

https://app.circleci.com/pipelines/github/clojure-emacs/enrich-classpath/120/workflows/18be8808-a406-4357-9c8e-0a2b304ab0fb/jobs/1503