Centralize duplicated queue/test helpers (clearOutbox, flushSpans, eventually/expectEventually, meaningful)

[sjvans]

Several test helpers are copy-pasted across many test files. Centralize them into a shared test-utils module (e.g. test/bookshop/lib/queue-test-utils.js or test/_utils.js) and import from there.

Duplicated helpers (current spread)

• eventually / expectEventually — the same force-flush-then-poll-assertion pattern under two names: eventually (span/tracing) in tracing-outboxed-batch, tracing-scheduled, tracing.test, tracing-messaging.js; expectEventually (metrics) in metrics-outbox, metrics-outbox-multitenant, metrics.test. Should be one helper (parameterize the flush fn).
• flushSpans — tracing-outboxed-batch, tracing-scheduled, tracing.test, tracing-messaging.js (unwraps the ProxyTracerProvider → delegate, guards the no-op).
• clearOutbox — metrics-outbox, tracing-messaging.js (timeout-bounded DELETE FROM cds.outbox.Messages).
• meaningful / isOutboxScanTrace — tracing-messaging* (×3 via the shared helper), tracing.test, tracing-scheduled, tracing-messaging.js (filters bookkeeping outbox-scan traces).

Why

These landed independently while stabilizing the HANA suite (#481), so the same logic now exists in 2-6 copies — a maintenance and drift hazard (a fix to the poll/flush logic must be applied N times). Centralizing also shrinks the per-file boilerplate.

Notes

• Keep the shared module dependency-light and consistent with MyInMemorySpanExporter / MyInMemoryMetricReader conventions (must not require('@sap/cds') at module top).
• Reconcile the metric-flush (forceFlush on the meter provider) vs span-flush (flushSpans on the tracer provider) variants — likely one generic eventually(assertion, {flush}) plus thin flushSpans/flushMetrics wrappers.

Part of the test-quality cluster (#477 / #478 / #475 / #486 / #487).

[sjvans]

another helper introduced by #491: asExternalClient

[sjvans]

Done via #492 (merged to develop 65157bc): 6 helpers (clearOutbox, flushSpans, eventually, makeExpectEventually, asExternalClient, isOutboxScanTrace/meaningful) centralized into test/utils.js; ~10 suites deduped (net −133 lines). Behavior-preserving (sqlite 65/12 unchanged), independently reviewed.