Show how to disable system assert during testing (#93971)

We only show how to disable user asserts during testing, we should show
how to disable system asserts too.

Author: iverase

TESTING.asciidoc

@@ -70,7 +70,7 @@ When running Elasticsearch with `./gradlew run`, assertions are enabled by
 default. To disable them, add the following command line option:
 
 -------------------------
--Dtests.jvm.argline="-da"
+-Dtests.jvm.argline="-da -dsa"
 -------------------------
 
 ==== Distribution