chore(ci): do not run connectivity-tests as unit tests (#1129)

Some macOS machines in CI have currently started using npm 7 by
default. This has the consequence that npm complains about a
missing `test` script in the connectivity-tests package.

Since we eventually want to migrate to npm 7, we address this
by adding a `test` script (and removing connectivity-tests
from the unit test matrix), rather than enforcing usage of npm 6
everywhere.

Author: addaleax