Jetty 12 : Review usages of `WebAppContext.getWebInf()`

[joakime]

Jetty version(s) Jetty 12

Description There are a few places that use WebAppContext.getWebInf() that seem to assume that there is only possible to have 0 or 1 WEB-INF Resource. Review usages (and add test cases where missing) to ensure that we can handle multiple WEB-INF resources (via a ResourceCollection) correctly.

The javadoc will be updated from an unrelated PR to read ...

        // Is there a WEB-INF directory anywhere in the Resource Base?
        // ResourceBase could be a ResourceCollection
        // The result could be a ResourceCollection with multiple WEB-INF directories
        // Can return from WEB-INF/lib/foo.jar!/WEB-INF locations
        // Can also return from a META-INF/versions/#/WEB-INF locations