test: mark test-buffer-tostring-range as flaky

Author: marco-ippolito

test/parallel/parallel.status

@@ -79,6 +79,8 @@ test-domain-no-error-handler-abort-on-uncaught-8: PASS, FLAKY
 test-domain-no-error-handler-abort-on-uncaught-9: PASS, FLAKY
 test-domain-throw-error-then-throw-from-uncaught-exception-handler: PASS, FLAKY
 test-domain-with-abort-on-uncaught-exception: PASS, FLAKY
+https://github.com/nodejs/node/issues/56726
+parallel/test-buffer-tostring-range: PASS
 
 [$system==freebsd]