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

Author: marco-ippolito

test/parallel/parallel.status

@@ -70,6 +70,8 @@ test-domain-with-abort-on-uncaught-exception: PASS, FLAKY
 test-tick-processor-arguments: SKIP
 # https://github.com/nodejs/node/issues/54534
 test-runner-run-watch: PASS, FLAKY
+https://github.com/nodejs/node/issues/56726
+parallel/test-buffer-tostring-range: PASS, FLAKY
 
 [$system==freebsd]