Fix Gradle plugin logic for removing provided artifacts

The new ResolvedArtifact-based minus implementation was checking the
wrong Set when deciding whether or not a ResolvedArtifact should
be included in the result. This was leading to provided dependencies,
that should have only been packaging in WEB-INF/lib-provided also
being packaged in WEB-INF/lib.

The WarPackaging tests have been updated. In addition to checking that
WEB-INF/lib-provided does not contain anything that’s unexpected, they
also verify the contents of WEB-INF/lib

Fixes #1187