Blender Git Loki

Tests: speed up render tests by running multiple in the same process

Blender startup time and shader compilation is a big factor when running
hundreds of tests, so now all renders in the same ctest run in the same
process.

This was previously reverted due to skipping other tests when one test
crashed. Now if a test crashes, Blender is re-run with the remaining
tests so we get results from them still.