rendering/pixeldata.py skipped tests are darker in parallel

[JustinPrivitera]

Describe the bug

These tests are in the skip list for all platforms because when run in parallel, the images come out darker. Even when run in parallel with just one process.

Expected behavior

This should be identical to the serial version of the tests.

Desktop

• OS and version: LC toss3
• VisIt Version: develop

[markcmiller86]

The testing @JustinPrivitera did here included a case where we ran parallel (not scalable or iceT) on a single processor...and still got different results from serial. This lead to the idea of another kind of test mode...we run single processor parallel and ensure it generates the same results as serial. If it does not, it suggests there are parallel code blocks that do not match their serial counterparts.