test(runner): apply per-family nextest config and a thread count to runner rows #5295

User Story

I maintain OpenShell's end-to-end tests. When a family of tests moves from e2e/rust onto testkit, the tmachine runner has to give it the timeouts and concurrency the old lanes gave it, so a ported test doesn't pass in the old lane and time out in the runner.

Description

The runner runs every family with one fixed nextest profile: 30 s slow-timeout × 4 and one test thread. The old e2e/rust lanes get more from the repository's .config/nextest.toml: per-test slow-timeout overrides and test groups such as kubernetes-ha and kubernetes-shared-supervisor-pool. The runner passes its own --config-file, so that file doesn't apply to runner rows.

Add the runner side of this when the first batch that needs it is ready:

• Pass the runner's openshell profile as a nextest tool config (--tool-config-file openshell-test-runner:<path>) instead of --config-file, so a family's own config is read.
• Have the bundle carry tests/suites/<family>/.config/nextest.toml into the remapped workspace, so a family can set per-test timeouts and test groups, or override the profile.
• Add a --test-threads N runner flag. The default of 1 keeps today's serial --no-capture run. Parallel runs lose streamed output, so document that.
• Add a test_threads column to the e2e-suites.yml rows so CI can set it.

An implementation was written and reviewed in #5277 (commit 8441388e5, reverted before merge to keep that PR small). Start from it.

Context

Likely first users:

• The Kubernetes batch: shared-supervisor, HA and rebalancing tests need the existing test groups and longer timeouts.
• credential_gating after test(e2e): split credential_gating into independent parallel tests #5264 splits it: independent tests with unique names can run in parallel.
• Any common batch that wants to run in parallel so lanes don't get slower as tests move in.
• settings_management::settings_global_override_round_trip, which PR1 (test(conformance): port sandbox lifecycle and API e2e tests to testkit #5431) left in e2e/rust. It changes a gateway-wide setting, so it needs a serial test group before it moves to conformance settings. sandbox_setting_round_trip also clears and restores the gateway-wide ocsf_json_enabled, so it belongs in the same group.

Not needed by the first conformance, Podman or Docker batches if they run serially under the default profile.

Part of #5281.

Definition of Done

• A family can set per-test timeouts and test groups through tests/suites/<family>/.config/nextest.toml, and they apply under the runner.
• --test-threads is configurable from the runner and from the e2e-suites.yml rows; the default is unchanged.
• Runner unit tests cover the nextest arguments and the family config copy.
• TESTING.md documents the behavior.