display-test: Fix a race condition in test suite

Several tests in this suite use setting and checking client.display_stopped (in
test-compositor.h) to synchronise between threads. This is a data race because
display_stopped is a non-atomic int. Fix this by making it an atomic_bool
instead. We don't need to change the access code because reads and writes are
sequentially consistent by default.

This can be reproduced (with both clang and gcc) by running
```
meson -Db_sanitize=thread build
cd build
ninja
meson test
```

Signed-off-by: Fergus Dall <sidereal@google.com>