test(core-consent): add missing test coverage and fix diff allowlist

Add symbols.test.ts to cover CONSENT_SYMBOLS constant.
Add recording-consent.test.ts to core-testing (follows the pattern
of all other recording doubles which each have a sibling test file).
Refactor payload-consent.test.ts to extract a makeConsent() helper,
reducing clone groups from 7 to 2.
Add new test to cover grantedAt=null branch in deserializeEntry (withdraw
before any grant → persisted as null → loaded as undefined).
Extend coverage/diff.mjs allowlist for packages/core-testing/ since that
tooling package does not install @vitest/coverage-v8 and therefore produces
no lcov data.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

coverage/summary.json

@@ -1,17 +1,17 @@
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-  "generatedAt": "2026-05-19T12:37:46.347Z",
-  "commit": "5792b74",
+  "generatedAt": "2026-05-19T12:48:28.197Z",
+  "commit": "7dd46b6",
   "repo": {
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@@ -59,16 +59,16 @@
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