I don't want to waste any more of my 10 appspot.com subdomains for test projects. Is there an elegant way to have multiple appengine apps, each to its own folder, e.g. xx.appspot.com/myapp1, xx.appspot.com/myapp2? I want the configuration to be identical or very close to the actual deployment configuration at a subdomain's root.
What you could maybe do is (ab-)use the different versions for completely different projects. In this case you do not have to change anything in web.xml.
http://1.latest.xx.appspot.com for myapp1
http://2.latest.xx.appspot.com for myapp2
They do share the same datastore and memcache and task queues, but you would have the same situation with separating the paths. And if the table names do not collide, that should not be a problem.
You do get different admin consoles (log viewers etc) for each version.
You could always create a new Google account and get 10 new apps, right?
You could use dummy Google accounts for your test projects, and add your original Google account as a developer to a project you decide to actively work on... That way your original Google account doesn't get "polluted" by test projects.
Just ask for more apps. Google would rather give you more apps than have you work around the limit. See here