Fixes for the 3-way comparison in tests_to_html.py

[krohmerNV]

Tried to do a comparison between MDL 1.6, 1.7, and 1.8 and found two issues:

• the computed file paths are not correct when passing different --inputdir1, --inputdir2, and --inputdir3
• when comparing basically mdl against mdl and mdl, the file names of the diff images collide

[jstone-lucasfilm]

@krohmerNV I like the idea of this improvement, though in my local tests it seems to break the simpler two-way comparisons between GLSL and OSL:

[krohmerNV]

I'll investigate, thanks!