Add new test for waiting on pending calls in threads

Add new test for waiting on pending calls in threads

Based on code contributed by Manish Narang. This is not included in the
automated test suite, because it isn't reliable on heavily-loaded
automatic test infrastructure like Travis-CI.

Reviewed-by: Philip Withnall <withnall@endlessm.com>
[smcv: Add the test to the CMake build system too, as requested]
[smcv: Convert into a manual test]
Signed-off-by: Simon McVittie <smcv@collabora.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=102839
(cherry picked from commit 0b1e2928606e0cf138c38e75eb248d2ed19bff15)