This is a followup to JDK-8336332

I'd like to clean up the "expected" stderr output coming from:
CssParser tests
Snapshot1Test

and some other places.

see also https://github.com/openjdk/jfx/pull/1897#pullrequestreview-3241971053

NOTE:
The phrase "Exception is snapshot callback" , coming from Node:2421 and Scene:1612, is interpreted as a legitimate exception in Snapshot1Test. Before it gets resolved in JDK-8320266 , we could simply rephrase the message or insert an extraneous symbol before the word "Exception" so as not to trip the regex.