nixos/tests/plasma5: cannot match username due to sddm changes

[worldofpeace]

Recent changes to sddm theming in plasma5 make detecting the username nonfunctional, despite being more visually appealing.
Any reasonable way to improve contrast would hopefully make the detection work.
@samueldr suggested perhaps using a different sddm theme with better contrast.
And perhaps the blur could be interfering also.

Here is sddm before the 5.13 release:

Here is after:

cc @fpletz @globin @bkchr @ttuegel

Also thanks to @grahamc for IRC hand holding.