Use a non-greedy quantifier:
test:(.*?),
Or a character class:
test:([^,]*),
To ignore the comma as well:
test:([^,]*)
If you'd like to omit the test:
as well you can use a look-behind like this:
(?<=test:\s)[^,]*
Since you're using this grok debugger, I was able to get this to work by using a named capture group:
(?<test>(?<=test:\s)[^,]*)