tests: add reusable failpoint-enabled go test script for local workflow

## Enhancement

The current failpoint-enabled unit test guidance in `docs/agents/testing-flow.md` uses an inline shell block. That makes the workflow harder to reuse and awkward to parameterize when a developer needs to pass additional `go test` flags or a different tag set such as `nextgen`.

### Proposed change

- add a reusable script under `tools/check/` to wrap `make failpoint-enable`, `go test`, and `make failpoint-disable`
- accept the package path plus arbitrary `go test` arguments
- default to `-tags=intest,deadlock` when `-tags` is omitted
- allow callers to override tags explicitly, for example `-tags=intest,deadlock,nextgen`
- update `docs/agents/testing-flow.md` to reference the script instead of duplicating the subshell inline

### Benefit

This keeps the failpoint workflow consistent, reduces copy/paste errors, and makes ad hoc local test runs easier to execute.