bigtable/bttest: GC'd memory is not released

[philpearl]

Client

Bigtable bttest emulator

Environment

Docker container running either on a Mac or in GKE

Go Environment Still relevant with Go 1.18

Code

Expected behavior

If we add data to the BigTable emulator over time, but we have a GC policy in place that should keep the amount of data in the tables down to a reasonable level, the emulator should keep running

Actual behavior

Memory usage keeps expanding to the point the emulator hits an OOM condition

Screenshots

No screenshots

Additional context

See https://github.com/googleapis/google-cloud-go/pull/3930