I think this sometimes happens in travis. I can reproduce locally as follows:
$ for i in {0..1000}; do echo "Test run: $i"; src/test/test_bitcoin --log_level=ALL --run_test=scheduler_tests;done
Output:
[...]
Test run: 7
Running 1 test case...
Entering test suite "Bitcoin Test Suite"
Entering test suite "scheduler_tests"
Entering test case "manythreads"
test/scheduler_tests.cpp(70): info: check nTasks == 0 passed
test/scheduler_tests.cpp(82): info: check nTasks == 100 passed
test/scheduler_tests.cpp(83): info: check first < last passed
test/scheduler_tests.cpp(84): info: check last > now passed
test/scheduler_tests.cpp(113): info: check counter[i] != 0 passed
test/scheduler_tests.cpp(113): info: check counter[i] != 0 passed
test/scheduler_tests.cpp(113): info: check counter[i] != 0 passed
test/scheduler_tests.cpp(113): info: check counter[i] != 0 passed
test/scheduler_tests.cpp(113): info: check counter[i] != 0 passed
test/scheduler_tests.cpp(113): info: check counter[i] != 0 passed
test/scheduler_tests.cpp(113): info: check counter[i] != 0 passed
test/scheduler_tests.cpp(113): info: check counter[i] != 0 passed
test/scheduler_tests.cpp(113): info: check counter[i] != 0 passed
test/scheduler_tests.cpp(113): info: check counter[i] != 0 passed
test/scheduler_tests.cpp(116): info: check counterSum == 200 passed
Leaving test case "manythreads"; testing time: 4653mks
Leaving test suite "scheduler_tests"
Leaving test suite "Bitcoin Test Suite"
*** No errors detected
Test run: 8
Running 1 test case...
Entering test suite "Bitcoin Test Suite"
Entering test suite "scheduler_tests"
Entering test case "manythreads"
test/scheduler_tests.cpp(70): info: check nTasks == 0 passed
test/scheduler_tests.cpp(82): info: check nTasks == 100 passed
test/scheduler_tests.cpp(83): info: check first < last passed
test/scheduler_tests.cpp(84): info: check last > now passed
^C
