Add remote debugger instructions to TESTING.asciidoc

TESTING.asciidoc

@@ -25,6 +25,12 @@ run it using Gradle:
 gradle run
 -------------------------------------
 
+or to attach a remote debugger, run it as:
+
+-------------------------------------
+gradle run --debug-jvm
+-------------------------------------
+
 === Test case filtering.
 
 - `tests.class` is a class-filtering shell-like glob pattern,