test: fix flaky interval test
A test was recently introduced that consistently fails on OS X & a few other machines. This PR reliably fixes it.
CI: https://ci.nodejs.org/job/node-test-pull-request/12522/ Stress test CI: https://ci.nodejs.org/job/node-stress-single-test/1686/
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
test