refactor replicator tests

[torcolvin]

cleanup/prep for active replicator tests in #7201

Created helper functions to write ActiveReplicator tests with less boilerplate.

CBG-4370 create a cancel context inside BlipSyncContext

Pre-review checklist

• Removed debug logging (fmt.Print, log.Print, ...)
• Logging sensitive data? Make sure it's tagged (e.g. base.UD(docID), base.MD(dbName))
• Updated relevant information in the API specifications (such as endpoint descriptions, schemas, ...) in docs/api

Integration Tests

• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGateway-Integration/2814/

[torcolvin]

Decided not to pursue this because t.Cleanup(http.Server.Close) will run after the defer for the rest tester, and this doesn't behave in predictable ways.