CompositorPerformance.regression_test_1563287 leaves orphan clients running

[AlanGriffiths]

$ mir_performance_tests --gtest_filter=CompositorPerformance.regression_test_1563287
Running main() from main.cpp
Note: Google Test filter = CompositorPerformance.regression_test_1563287
[==========] Running 1 test from 1 test case.
[----------] Global test environment set-up.
[----------] 1 test from CompositorPerformance
[ RUN      ] CompositorPerformance.regression_test_1563287
[2020-07-28 16:06:41.108051] < -warning- > gbm-kms: Unable to determine the current display mode.
[2020-07-28 16:06:41.108118] < -warning- > gbm-kms: Unable to determine the current display mode.
[2020-07-28 16:06:41.108151] < -warning- > gbm-kms: Unable to determine the current display mode.
[2020-07-28 16:06:41.108181] < -warning- > gbm-kms: Unable to determine the current display mode.
[2020-07-28 16:06:41.108211] < -warning- > gbm-kms: Unable to determine the current display mode.
[2020-07-28 16:06:41.147676] < -warning- > gbm-kms: drmModeCrtcSetGamma failed: Invalid argument
[       OK ] CompositorPerformance.regression_test_1563287 (12943 ms)
[----------] 1 test from CompositorPerformance (12943 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test case ran. (12943 ms total)
[  PASSED  ] 1 test.
$ ps -eaf | grep mir_demo
alan     12945     1 37 16:06 pts/3    00:00:06 /usr/bin/mir_demo_client_wayland
alan     12951     1 39 16:06 pts/3    00:00:06 /usr/bin/mir_demo_client_wayland
alan     12957     1 39 16:06 pts/3    00:00:06 /usr/bin/mir_demo_client_wayland
alan     12965  2825  0 16:06 pts/3    00:00:00 grep --color=auto mir_demo