I’ve been working with a large scale legacy system (in the Michael Feathers’ sense). Strangely enough there are a small handful of tests, unfortunately most of them falling by the way side having not been added into an Continuous Integration process. Many of them no longer look relevant, others, unable to run without a specific person’s machine (due to configuration files or special databases). Others look like a mechanism for people to simply diagnose issues with plenty of tracing statements. All of this was an inevitable system without someone running all the tests all the time.
Any serious team serious about continuous integration will think hard about their development testing strategy, considering approaches that allow you to run tests all the time. The strategy needs to consider a good way of preventing side effects from other tests (global state, global configuration, or left over state in external resources) balancing out speed with good quality feedback. Staggering build processes into smaller chunks, such as using build pipelines are a great way of achieving this.
Please don’t let leave your tests to die a sad and lonely death. It will take discipline and effort to keep them running yet the results will often pay back for themselves very quickly on any system with a reasonable lifespan.