deft test: don't run a silly default library

```console
18:55:35 ~/dylan/workspaces/json (dev*) 
$ deft test
Open Dylan 2026.1

Opened project testworks-test-suite-app (/Users/cgay/dylan/workspaces/json/_packages/testworks/3.4.0/src/tests/testworks-test-suite-app.lid)
```

Why are we running `testworks-test-suite-app` rather than `json-test-suite`?  Must be a problem with `find-active-package-test-libraries`.