tests: Remove buffer-count

tests: Remove buffer-count

buffer-count was introduced in line with a Mesa change which forced
an earlier block on frame events to try to enforce double-buffering
where available.

The Mesa change has since been reverted (Mesa commit 9ca6711faa), as
this had unpleasant interactions with buffer_age in particular, so this
test is no longer valid.

Additionally, it only worked on backends which initialised EGL (not
headless-backend, where tests generally run), which can be flaky due to
initialisation races. Not only that, but on the DRM backend, we can
legitimately enter triple-buffering due to promoting the surface to a
hardware plane, skipping GPU composition.

In light of all this, just remove the test.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>