fix(node): LocalVariables integration should use setupOnce

[timfish]

Closes #10278

This integration incorrectly used setup which then caused issues when the core code was changed. It should have been using setupOnce.

Unfortunately, setupOnce causes issues with the unit tests since it then relies on getClient to get the client instance for the client options. I've tried using jest to mock getClient but this doesn't work which is likely due to installedIntegrations.

Because we already have extensive integration tests for LocalVariables I have removed the unit tests because they are becoming a maintenance burden and actually test less than the integration tests.

After v8, I plan to move Local variables lookup to the worker thread at which point we can revisit how these might be better tested in unit tests.