EventTest is failing on Mac (mutex lock failed: Invalid argument)

[j-rivero]

Following the comment on https://github.com/osrf/gazebo/pull/3042#issuecomment-896201207. The EventTest after changes in #3042 is failing on Mac.

25: [ RUN      ] EventTest.RaceConditions
25: Start create connections thread
25: Start access thread
25: Start remove thread
25: libc++abi.dylib: terminating with uncaught exception of type std::__1::system_error: mutex lock failed: Invalid argument
 25/416 Test  #25: UNIT_Event_TEST .......................................Subprocess aborted***Exception:   3.42 sec
test 26
        Start  26: check_UNIT_Event_TEST

26: Test command: /Users/jenkins/workspace/gazebo-ci-pr_any-homebrew-amd64/gazebo/tools/check_test_ran.py "/Users/jenkins/workspace/gazebo-ci-pr_any-homebrew-amd64/build/test_results/UNIT_Event_TEST.xml"
26: Test timeout computed to be: 10000000
26: Checking for test results in /Users/jenkins/workspace/gazebo-ci-pr_any-homebrew-amd64/build/test_results/UNIT_Event_TEST.xml
26: Cannot find results, writing failure results to /Users/jenkins/workspace/gazebo-ci-pr_any-homebrew-amd64/build/test_results/UNIT_Event_TEST.xml
 26/416 Test  #26: check_UNIT_Event_TEST .................................***Failed    0.06 sec

I was unable to trigger the problem on Linux when creating the test and trying to reproduce race conditions but seems like Mac could be affected (or the test implementation is buggy).