Well, gets kind of tricky because of the arbitrary number of lines between "test" and "world", plus the arbitrary number of ines between "world" and "test"...
.,.. someone smarter than me will probably give you a better answer, but this works
Code:
sed 's/$/ @/' file.txt | sed -n '/test/,/world/p' | sed '/world/G' | sed -e :a -e '/@$/N; s/\@\n//; ta'
