Intermittent hang & fail in mir_acceptance_tests.TestClientCursorAPI.*

I'm building on 64bit trusty, error happens with both g++ and clang++

Doesn't happen often - maybe twice in past day.

        Start 25: mir_acceptance_tests.TestClientCursorAPI.*
 25/204 Test #25: mir_acceptance_tests.TestClientCursorAPI.* .................................***Failed 60.02 sec
...
99% tests passed, 1 tests failed out of 204

Total Test time (real) = 74.45 sec

The following tests FAILED:
  25 - mir_acceptance_tests.TestClientCursorAPI.* (Failed)
Errors while running CTest

~~~~

With a bit of investigation using:

$ bin/mir_acceptance_tests --gtest_filter=TestClientCursorAPI.* --gtest_break_on_failure --gtest_repeat=100
...
Repeating all tests (iteration 8) . . .
...
unknown file: Failure
C++ exception with description "Timeout while waiting for child to change state" thrown in TearDown().
Segmentation fault (core dumped)
....
Repeating all tests (iteration 40) . . .
...
unknown file: Failure
C++ exception with description "Timeout while waiting for child to change state" thrown in TearDown().
Segmentation fault (core dumped)
...
Repeating all tests (iteration 88) . . .
...
unknown file: Failure
C++ exception with description "Timeout while waiting for child to change state" thrown in TearDown().
Segmentation fault (core dumped)
...
Repeating all tests (iteration 12) . . .
...
unknown file: Failure
C++ exception with description "Poll on readfd for pipe timed out" thrown in the test body.
unknown file: Failure
C++ exception with description "Timeout while waiting for child to change state" thrown in TearDown().
Segmentation fault (core dumped)

So not too hard to reproduce.