Make point cloud attenuation tests less fragile

See @likangning93's comment in https://github.com/AnalyticalGraphicsInc/cesium/pull/7235:

> Can confirm the fix as well, but the problem is more that the underlying spec is fragile, which is my fault from when these were written... the "reference" noAttenuationPixelCount value is hardcoded when it should probably be computed before any specs are run instead.
>
> Maybe then we won't have to tune parameters to get this to pass both on Linux and Windows?

Read the comment in pull request #7235 first, then locate the point cloud attenuation specs and their hard-coded noAttenuationPixelCount value. Done means the reference count is computed before the specs run and the tests no longer need platform-specific parameter tuning on Linux and Windows.

Written by the indexing model from the issue text.