kvserver: refactor v1 flow control integration tests for v2

The existing flow control integration tests, in
`flow_control_integration_test.go`, provide a substantial suite of tests
to verify flow control integration behavior.

Refactor the `flowControlTestHelper` and associated use in tests, to
enable sharing the helper between v1 (existing) and not-yet implemented
v2 flow control integration tests.

Part of: #130187
Release note: None