Adding example of running tests without a snapshot build (elastic#65899)

It can be difficult to know how to run release builds. Certain changes require that tests run without a snapshot build (consider testing a feature behind a feature flag).

This adds a small doc line specifying what args to pass to run tests without a snapshot build.

TESTING.asciidoc

@@ -248,6 +248,13 @@ Pass arbitrary jvm arguments.
 ./gradlew test -Dtests.jvm.argline="-Djava.security.debug=access,failure"
 ------------------------------
 
+Pass build arguments.
+
+------------------------------
+# Run tests against a release build. License key must be provided, but usually can be anything.
+./gradlew test -Dbuild.snapshot=false -Dlicense.key="x-pack/license-tools/src/test/resources/public.key"
+------------------------------
+
 == Running verification tasks
 
 To run all verification tasks, including static checks, unit tests, and integration tests: