test: move handler handler tests to testcontainers db (#1782)

## Summary
- switch HTTP handler tests from the in-memory fake database client to
the shared Postgres testcontainers setup
- add a shared database-backed test helper for the handlers test package
- delete the unused fake database implementation

## Testing
- go test ./core/internal/httpserver/handlers/...

Signed-off-by: Eitan Yarmush <eitan.yarmush@solo.io>

Author: EItanya