[docs] Migrate unit test docs

Author: andrewnicols

data/migratedPages.yml

@@ -1806,6 +1806,9 @@ Web_services_API:
 Web_services_files_handling:
 - filePath: "/docs/apis/subsystems/external/files.md"
   slug: "/docs/apis/subsystems/external/files"
+Writing_PHPUnit_tests:
+- filePath: "/docs/guides/testing/index.md"
+  slug: "/docs/guides/testing/"
 Writing_acceptance_tests:
 - filePath: "/general/development/tools/behat/writing.md"
   slug: "/general/development/tools/behat/writing"