Previously, if getValidDocumentRoot() returned null, only the embedded
Tomcat container would use a temporary directory. This left Jetty and
Undertow unable to provide a URL for the root of the servlet context,
i.e. servletContext.getResource("/") would return null.

This commit updates the embedded containers for Jetty and Undertow to
behave in the same way as Tomcat. A test has been added to verify that
all three containers can produce a non-null URL for the root of the
servlet context.

Closes gh-2878