docs: prefer `test` rather than `t`

[chadoh]

AVA's docs tell people to use t, partially because the top-level variable is usually named test. But in our context we have no such top-level variable, and we have a lot teach to people who are so new to all of it, so it makes more sense to use a longer variable name.

[chadoh]

@willemneal near-sandbox-install test still failing, and this is branched of latest main.

TypeError {
  message: 'Cannot read properties of undefined (reading \'close\')',
}

I'm going to merge this, since this branch did not affect this behavior, but it looks like there's still work to do with the sandbox install.