Tests: add Eevee reference images, and add workbench tests

Being able to compare Eevee reference images is useful for refactoring I'm
working on so might as well add them now, even if we can still improve them.

Workbench tests are just rendering the same files as Cycles and Eevee. This
doesn't really tests many workbench settings until we add tests specifically
for them, but does cover how it it handles the different object types.

tests/python/cycles_render_tests.py

@@ -106,6 +106,7 @@ def main():
     from modules import render_report
     report = render_report.Report("Cycles Test Report", output_dir, idiff)
     report.set_pixelated(True)
+    report.set_reference_dir("cycles_renders")
     report.set_compare_engines('cycles', 'eevee')
     ok = report.run(test_dir, render_file)