fix(plugin): remove broken TESTING.md reference from README

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bgagent

abca-plugin/README.md

@@ -101,8 +101,6 @@ MISE_EXPERIMENTAL=1 mise tasks --all 2>/dev/null | grep -E '(build|install|compi
 cd cli && node lib/bin/bgagent.js submit --help && node lib/bin/bgagent.js list --help
 ```
 
-Full test results are documented in [`TESTING.md`](./TESTING.md).
-
 ## Development
 
 To modify the plugin: