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. If a test crashes, the remaining tests in the same category will
be marked as skipped.
Benchmarked on a quad core with ctest -j8.
cycles: 118.1s -> 94.3s
eevee: 66.2s -> 29.2s
workbench: 31.7s -> 8.6s