compiletest: error emitter pretty svg tests don't normalize color differences between Windows and Unixes

[jieyouxu]

This is the svg snapshots of the same test test/ui/error-emitter/multiline-removal-suggestion.rs (the former is the reference Unix-blessed snapshot, the latter is generated on Windows with //@ only-linux removed), but I believe this fails on Windows because the exact colors chosen are different. I believe this is because there are certain cmd.exe default bright colors which are very hard to read, so we actually pick different colors on purpose.

cc @estebank for FYI: I reasoned about the failure in #134664 incorrectly, I found out today after revisiting #132752, which is a different bug.