Added 4 active-selection-* tests

[TalbotG]

active-selection-021.html
reference/active-selection-021-ref.html

active-selection-024.html
reference/active-selection-024-ref.html

active-selection-031.html
reference/active-selection-031-ref.html

active-selection-045.html

support/100x100-red.png

This is a followup to
#20931

[TalbotG]

:( I am going to try again with 2 smaller PRs.

[TalbotG]

active-selection-045.html is most likely the test
that makes the check wpt-chrome-dev-stability fails.
Now, the only declaration that could affect Chrome is
font-size: 300px .
I am carefully checking again this in
#24723
and if the check wpt-chrome-dev-stability fails
again over there, then I will try again with
font-size: 150px .

When I try active-selection-045.html with
Chromium 86.0.4211.0 (dev build)
with a font size of 300px,
I see/get no problem at all.

[TalbotG]

I must close this PR and delete this branch now that
#24713
#24714
have merged.