basic/terminal-util: rename our replacement highlight-yellow and test both the...

basic/terminal-util: rename our replacement highlight-yellow and test both the original and replacement

The fact that we cannot use the "real" yellow color and need to use some other
color is essentially a bug. In this particular case the issue is in urxvt, see
a9009769. While we work around this by using a
different color, let's not forget that this is just a (hopefullly temporary)
workaround.

https://bugzilla.redhat.com/show_bug.cgi?id=1859886
https://bugzilla.redhat.com/show_bug.cgi?id=1859889
https://bugzilla.redhat.com/show_bug.cgi?id=1859894