update to tav@5

I'd like to add some conveniences to the [tav](https://github.com/watson/test-all-versions) tool we use for some tests. Currently we're on tav@4, but current dev is tav@5. That means moving past this breaking change:

```
commit d92db14ec244ff6abb1a17624a368166c7302666
Author: Thomas Watson
Date: 2020-05-12T08:59:23+02:00 (6 months ago)

feat!: support an array of test cases for a module

BREAKING CHANGE: The root keys now all have to match a valid module name
and the custom `name` property is no longer supported.
```

which shouldn't be hard, just busy work.

- Note the new "env:" featre incase that would help things.