Pass --debug to eliminate trace warnings on JRuby

power_assert enables traces, which are only partially supported on
JRuby without --debug. The warning that results breaks
assert_separately due to stderr output from the child process.

test/lib/core_assertions.rb

@@ -143,6 +143,8 @@ class Test::Unit::Runner
 eom
         args = args.dup
         args.insert((Hash === args.first ? 1 : 0), "-w", "--disable=gems", *$:.map {|l| "-I#{l}"})
+        # JRuby needs --debug for trace support in power_assert
+        args.push("--debug") if RUBY_ENGINE == 'jruby'
         stdout, stderr, status = EnvUtil.invoke_ruby(args, src, true, true, **opt)
         abort = status.coredump? || (status.signaled? && ABORT_SIGNALS.include?(status.termsig))
         assert(!abort, FailDesc[status, nil, stderr])