RenderTests: Fix Incorrect Command in Report

When running the render test for EEVEE-Next the command printed on
the report to update the reference images was incorrect. In stead of
displaying
`BLENDER_TEST_UPDATE=1 ctest -R eevee_next` it displayed
`BLENDER_TEST_UPDATE=1 ctest -R eevee next`.

The cause of this is that the title of the report is used to create
the command. EEVEE-Next has a space in its title which generates
an incorrect command.

A quick fix would be to replace spaces with underscores. But it would
be better to fix this more clearly by adding an attribute containing the
engine name.

This PR adds a `set_engine_name` method to the Report class. By
default the engine name is set based on the title.

Pull Request: https://projects.blender.org/blender/blender/pulls/117503

tests/python/eevee_next_render_tests.py

@@ -136,6 +136,7 @@ def main():
     from modules import render_report
     report = render_report.Report("Eevee Next", output_dir, oiiotool)
     report.set_pixelated(True)
+    report.set_engine_name('eevee_next')
     report.set_reference_dir("eevee_next_renders")
     report.set_reference_override_dir(reference_override_dir)
     report.set_compare_engine('cycles', 'CPU')