Investigate flaky test-util-sigint-watchdog on FreeBSD

[Trott]

I've seen this at least twice now, so it's definitely A Thing...

https://ci.nodejs.org/job/node-test-commit-freebsd/3505/nodes=freebsd10-64/console:

not ok 1046 parallel/test-util-sigint-watchdog
# 
# assert.js:89
#   throw new assert.AssertionError({
#   ^
# AssertionError: false === true
#     at Timeout.common.mustCall (/usr/home/iojs/build/workspace/node-test-commit-freebsd/nodes/freebsd10-64/test/parallel/test-util-sigint-watchdog.js:25:12)
#     at Timeout._onTimeout (/usr/home/iojs/build/workspace/node-test-commit-freebsd/nodes/freebsd10-64/test/common.js:411:15)
#     at tryOnTimeout (timers.js:232:11)
#     at Timer.listOnTimeout (timers.js:202:5)

/cc @addaleax

[Trott]

Another one (no new info):

https://ci.nodejs.org/job/node-test-commit-freebsd/3518/nodes=freebsd10-64/console

not ok 1048 parallel/test-util-sigint-watchdog
# 
# assert.js:89
#   throw new assert.AssertionError({
#   ^
# AssertionError: false === true
#     at Timeout.common.mustCall (/usr/home/iojs/build/workspace/node-test-commit-freebsd/nodes/freebsd10-64/test/parallel/test-util-sigint-watchdog.js:51:12)
#     at Timeout._onTimeout (/usr/home/iojs/build/workspace/node-test-commit-freebsd/nodes/freebsd10-64/test/common.js:411:15)
#     at tryOnTimeout (timers.js:232:11)
#     at Timer.listOnTimeout (timers.js:202:5)
  ---
  duration_ms: 0.644

[addaleax]

Proposed fix: #7933