ProcTest unit tests flaky

$ make check -j3
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[ RUN      ] ProcTest.MultipleThreads
../../../../3rdparty/libprocess/3rdparty/stout/tests/proc_tests.cpp:181: Failure
Value of: (procThreads).get()
  Actual: { 10050, 10053 }
Expected: childThreads
Which is: { 0 }
[  FAILED  ] ProcTest.MultipleThreads (2 ms)
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[ RUN      ] OsTest.children
../../../../3rdparty/libprocess/3rdparty/stout/tests/os_tests.cpp:361: Failure
Value of: children.get().size()
  Actual: 1
Expected: 0u
Which is: 0
../../../../3rdparty/libprocess/3rdparty/stout/tests/os_tests.cpp:379: Failure
Value of: children.get().size()
  Actual: 2
Expected: 1u
Which is: 1
[  FAILED  ] OsTest.children (31 ms)
[ RUN      ] OsTest.process
[       OK ] OsTest.process (0 ms)
[ RUN      ] OsTest.processes
[       OK ] OsTest.processes (11 ms)
[ RUN      ] OsTest.killtree
[       OK ] OsTest.killtree (64 ms)
[ RUN      ] OsTest.pstree
../../../../3rdparty/libprocess/3rdparty/stout/tests/os_tests.cpp:604: Failure
Value of: tree.get().children.size()
  Actual: 1
Expected: 0u
Which is: 0
-+- 10048 ./stout-tests 
 \--- 10050 ()
[  FAILED  ] OsTest.pstree (21 ms)
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[----------] Global test environment tear-down
[==========] 118 tests from 24 test cases ran. (480 ms total)
[  PASSED  ] 115 tests.
[  FAILED  ] 3 tests, listed below:
[  FAILED  ] ProcTest.MultipleThreads
[  FAILED  ] OsTest.children
[  FAILED  ] OsTest.pstree