The test spawns processes and tries to kill them, with timeouts and retry.
That is inherently racy, and it's hard to deterministically test the
interesting cases, without having unstable tests.

Try to adjust the timeout, to make it more stable:

    14:02:27 /builds/NetworkManager/NetworkManager/tools/run-nm-test.sh --called-from-make /builds/NetworkManager/NetworkManager/build    --launch-dbus=auto /builds/NetworkManager/NetworkManager/build/src/tests/test-core-with-expect
    --- stdout ---
    # random seed: R02S7748fae8fc946b7a755b72efb5815250
    1..5
    # Start of general tests
    ok 1 /general/nm_utils_monotonic_timestamp_as_boottime
    # NetworkManager-DEBUG: <debug> [1601992953.4091] kill child process 'test-s-1-3' (18615): sending SIGKILL...
    # NetworkManager-DEBUG: <debug> [1601992953.4242] kill child process 'test-s-1-3' (18615): waiting for process to terminate after sending no signal (0) and SIGKILL...
    # NetworkManager-DEBUG: <debug> [1601992953.4257] kill child process 'test-s-1-3' (18615): after sending no signal (0) and SIGKILL, process 18615 exited by signal 9 (20807 usec elapsed)
    Bail out! GLib:ERROR:../src/tests/test-core-with-expect.c:154:test_nm_utils_kill_child_sync_do: Did not see expected message NetworkManager-DEBUG: *<debug> [*] kill child process 'test-s-1-3' (*): waiting up to 1 milliseconds for process to terminate normally after sending no signal (0)...
    Bail out! test:ERROR:../src/tests/test-core-with-expect.c:457:test_nm_utils_kill_child: assertion failed (exit_status == 0): (6 == 0)
    --- stderr ---
    **
    GLib:ERROR:../src/tests/test-core-with-expect.c:154:test_nm_utils_kill_child_sync_do: Did not see expected message NetworkManager-DEBUG: *<debug> [*] kill child process 'test-s-1-3' (*): waiting up to 1 milliseconds for process to terminate normally after sending no signal (0)...
    **
    test:ERROR:../src/tests/test-core-with-expect.c:457:test_nm_utils_kill_child: assertion failed (exit_status == 0): (6 == 0)
    /builds/NetworkManager/NetworkManager/tools/run-nm-test.sh: line 279: 18325 Aborted                 "${NMTST_DBUS_RUN_SESSION[@]}" "${NMTST_LIBTOOL[@]}" "$NMTST_VALGRIND" --quiet --error-exitcode=$VALGRIND_ERROR --leak-check=full --gen-suppressions=all "${NMTST_SUPPRESSIONS[@]}" --num-callers=100 --log-file="$LOGFILE" "$TEST" "$@"