(Proc) Test fixtures

### What's hard to do? (limit 100 words)

It's useful to separate out tests for independent behaviors. Test procs require a lot of boilerplate in declaring channels, spawning the DUT proc, etc. This boilerplate needs to be added to every test proc, which is nominally fine through copy-paste, but for example, if you add a new channel to the DUT, now you need to separately update N test procs as well.

### Current best alternative workaround (limit 100 words)

Copy-paste. For example, if you add a new channel to the DUT, you can update the channel declaration and config for one test proc, then copy-paste that "header" to all the other test procs.

### Your view of the "best case XLS enhancement" (limit 100 words)

Some sort of test proc fixture `[#fixture]`?

Related, #1543 could allow different test procs to only specify the channels they use.