Test if servlet mapped to /* is invoked when request URI is / or /subdir/

We had a bug in CapeDwarf, which caused URIs that matched folders (that actually existed) to be served as though they were static files. These tests expose this type of bug.
Rename staticfiles_web.xml to web-overlapping-servlets-and-static-files.xml to be consistent

All appengine-web.xml files are called "appengine-web-*.xml", while all web.xml files are called "web-*.xml".
Test if servlet mapped to /* is invoked when request URI is /subdir (without trailing slash) and subdir is an actual directory in the WAR