tools: fix check-imports.py to match on word boundaries

`check-imports.py` was missing some unused `using` statements as it
was not matching on word boundaries (e.g. `MaybeLocal` was considered
a use of `Local`). Fix that and add some unit tests (which required
the script to be renamed to drop the `-` so it could be imported into
the test script).

PR-URL: https://github.com/nodejs/node/pull/33268
Refs: https://github.com/nodejs/node/issues/29226


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Christian Clauss <cclauss@me.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>