there are few jars like
appengine-testing.jar
appengine-local-rutime-shared.jar etc.
Basically when you create a new GAE project the eclipse plugin copies
some jars automatically in war folder, which are required when you
will be running ur application on GAE server, but it doesnt copy few
test jars which are required only for local dev env and testing.
So what i did i compared jars in my war folder and GAE SDK installed
folder. I found that few of jars not included so i included all in my
Eclipse build path/Junit run path(but didnt copy into war folder) and
that worked for me and then i didnt care to check which were the jars
actually needed and which were not as i included all jars.
But somehow in docs they have mentioned to included only one or two
jars and with these jars junit doesnt work.