docs(audience): add brief testMode mention to READMEs

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: bkbooth

packages/audience/pixel/README.md

@@ -152,6 +152,8 @@ Note: the nonce covers the inline snippet only. The CDN-loaded script (`imtbl.js
 | Safari | 14+ |
 | Edge | 80+ |
 
+Pass `"testMode": true` in the init options to mark all events with `test: true`, which lets you filter test traffic from production analytics.
+
 ## Documentation
 
 - [Tracking Pixel](https://docs.immutable.com/docs/products/audience/tracking-pixel) — this package (setup, consent modes, auto-tracked events)

packages/audience/sdk/README.md

@@ -61,6 +61,8 @@ audience.shutdown();
 </script>
 ```
 
+Pass `testMode: true` in the config to mark all events with `test: true`, which lets you filter test traffic from production analytics.
+
 ## Documentation
 
 - [Web SDK](https://docs.immutable.com/docs/products/audience/web-sdk) — API reference, usage, integration walkthrough