MasterTest.SlavesEndpointTwoSlaves is flaky

Observed on Arch Linux with GCC 6, running in a virtualbox VM:

[ RUN ] MasterTest.SlavesEndpointTwoSlaves
/mesos-2/src/tests/master_tests.cpp:1710: Failure
Value of: array.get().values.size()
Actual: 1
Expected: 2u
Which is: 2
[ FAILED ] MasterTest.SlavesEndpointTwoSlaves (86 ms)

Seems to fail non-deterministically, perhaps more often when there is concurrent CPU load on the machine.