test: make timers-blocking-callback more reliable

test-timers-blocking-callback may fail erroneously on
resource-constrained machines due to the timing nature of the test.

There is likely no way around the timing issue. This change tries to
decrease the probability of the test failing erroneously by having it
retry a small number of times on failure.

Tested on 0.10.38 (which has a bug that this test was written for) and
(modifying the test slightly to remove ES6 stuff) the test still seems
to fail 100% of the time there, which is what we want/expect.

PR-URL: https://github.com/nodejs/node/pull/14831
Fixes: https://github.com/nodejs/node/issues/14792


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>