[testsfail] ClientLatency failure under valgrind

Running the ClientLatency test suite under valgrind produces failures, which are not present on non-valgrind runs:

└>>> valgrind --trace-children=yes --leak-check=full --show-leak-kinds=definite --errors-for-leak-kinds=definite --track-fds=yes --num-callers=128 --suppressions=../tools/valgrind_suppressions_generic --suppressions=../tools/valgrind_suppressions_glibc_2.21 bin/mir_acceptance_tests --gtest_filter="ClientLatency*"
==1122== Memcheck, a memory error detector
==1122== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==1122== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==1122== Command: bin/mir_acceptance_tests --gtest_filter=ClientLatency*
==1122==
MIR_CLIENT_PLATFORM_PATH=bin/../lib/client-modules/
MIR_SERVER_PLATFORM_PATH=bin/../lib/server-modules/
LD_LIBRARY_PATH=/usr/local/lib:/usr/lib/debug:bin/../lib
exec=bin/mir_acceptance_tests.bin
==1122== Memcheck, a memory error detector
==1122== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==1122== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
==1122== Command: bin/mir_acceptance_tests.bin --gtest_filter=ClientLatency*
==1122==
Running main() from main.cpp
Note: Google Test filter = ClientLatency*
[==========] Running 2 tests from 1 test case.
[----------] Global test environment set-up.
[----------] 2 tests from ClientLatency
[ RUN ] ClientLatency.triple_buffered_client_uses_all_buffers
/home/kdub/source/mir/buffer-semantics/vault-disconnect/tests/acceptance-tests/test_latency.cpp:228: Failure
Value of: observed_latency
Expected: is > 1.9
  Actual: 1.9 (of type float)
[ FAILED ] ClientLatency.triple_buffered_client_uses_all_buffers (10030 ms)
[ RUN ] ClientLatency.throttled_input_rate_yields_lower_latency
/home/kdub/source/mir/buffer-semantics/vault-disconnect/tests/acceptance-tests/test_latency.cpp:259: Failure
Value of: observed_latency
Expected: is < 1.1
  Actual: 1.6 (of type float)
[ FAILED ] ClientLatency.throttled_input_rate_yields_lower_latency (2564 ms)
[----------] 2 tests from ClientLatency (12606 ms total)

[----------] Global test environment tear-down
[==========] 2 tests from 1 test case ran. (12649 ms total)
[ PASSED ] 0 tests.
[ FAILED ] 2 tests, listed below:
[ FAILED ] ClientLatency.triple_buffered_client_uses_all_buffers
[ FAILED ] ClientLatency.throttled_input_rate_yields_lower_latency