xds/clusterresolver: improve configbuilder tests by removing globals (grpc#8939)

Fixes grpc#8868.

## Summary
- Remove package-level test globals (`testLocalityIDs`,
`testResolverEndpoints`, `testEndpoints`, `testLocalitiesP0`,
`testLocalitiesP1`) and the `init()` function
- Add well-documented helper functions (`makeLocalityID`,
`makeEndpoint`, `makeResolverEndpoint`, `makeLocality`) so all test
state is local to each test function
- Add documentation to the existing `testEndpointWithAttrs` helper

RELEASE NOTES: N/A

Author: ulascansenturk