refactor: rephrase testing library description

Author: oleiade

docs/sources/k6/next/javascript-api/jslib/testing/_index.md

@@ -1,7 +1,7 @@
 ---
 title: 'testing'
 head_title: 'testing'
-description: 'k6 testing library for advanced assertions'
+description: 'The k6 testing library provides test assertion capabilities for both protocol and browser testing.'
 weight: 00
 ---
 

docs/sources/k6/next/using-k6/assertions.md

@@ -1,6 +1,6 @@
 ---
 title: 'Assertions'
-description: 'Advanced test assertions in k6 using the k6-testing library for both protocol and browser testing'
+description: 'Test assertions in k6 using the k6-testing library for both protocol and browser testing'
 weight: 04
 ---