Add experimental integration tests on core and exts

Would let us trigger mediawiki-core-extensions-integration (which tests
core+all wmf extensions) whenever one propose a patch on core or an
extension.

That is not going to have the attended effect on extensions that are not
part of the build. But we can figure that out later on.

Change-Id: I4149cb5905b80ddcca00ccbf985b4db497f20f27