www/firefox: depends on libXtst for the new libwebrtc

the current libwebrtc needs libXtst - even as firefox' docs only
"recommend" libXtst, it's actually a hard requirement.
this goes into bsd.gecko.mk, as it will affect all gecko ports in
due time, even if currently only www/firefox has the new libwebrtc.

PR: 261710
Reported by: Benjamin Takacs

(cherry picked from commit da0c0814149389ee67b37d6dabd791f65058b84a)