Add TESTING.md documenting test setup + caveats; reduce scattered comment clutter

[sjvans]

Add a top-level TESTING.md documenting the test setup and its caveats in one place, and trim the repeated explanatory comments now scattered across the test files.

Why

The test suite has accumulated a lot of hard-won, non-obvious knowledge that currently lives as long comment blocks duplicated across many files (~450+ comment lines in test/). The same caveats are re-explained repeatedly — e.g. the cds.spawn on sqlite skip rationale appears in ~7 places, the process.env.cds_requires_* config pattern in ~6, the Service Manager / multitenancy limitation in ~4. A single doc is easier to keep correct and lets the test files stay lean.

What TESTING.md should cover

• How to run: sqlite (default) vs HANA; the env the HANA path needs; npm test / vitest invocation.
• HANA specifics: single shared HDI container vs sqlite's per-file in-memory DB, and what that implies — serial file execution, outbox-settle in afterAll, retry: 2, raised timeouts (all gated on CI && HANA_DRIVER in vitest.config.mjs).
• Sanctioned skips/exclusions (the only two, per Eliminate all test skips and CI test-subset exclusions (sqlite + HANA) #477): SAP Passport (sqlite-skip) and multitenancy on HANA (needs a bound Service Manager; excluded from the HANA job). Anything else is debt.
• Test infrastructure: MyInMemorySpanExporter / MyInMemoryMetricReader (wired via .cdsrc.json profiles; must not require('@sap/cds') at module top; the reader mirrors production DELTA temporality), and the force-flush + poll (expectEventually / eventually) pattern for async span/metric assertions.
• Config gotchas: the process.env.cds_requires_* load-order sensitivity (see Tests configure CDS via process.env.cds_requires_* string-JSON instead of profiles/config #486), the TELEMETRY_TEST_HANA flag, and why config must precede any @sap/cds require.
• Known caveats: the pre-existing startup > NO_TELEMETRY=true local-env artifact, the internal-registry lockfile trap for @sap/* installs.

Then

Reduce the in-file comments to short pointers ("see TESTING.md — HANA shared-container caveats") rather than full re-explanations, keeping only what's needed to read the specific test.

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