Fix leaked processes in some platform agent tests

Fixed leaked processes recently introduced in platform agent tests (last observed in http://buildbot.oceanobservatories.org:8010/builders/coi_pycc/builds/2257/steps/sa/logs/stdio):

- TestPlatformAgent.test_resource_monitoring_recursion_parameter
- TestPlatformRobustness.test_with_intermediate_subplatform_directly_stopped

@edwardhunter please merge

@bobfrat fyi