tests: Tidy up cases in a switch statement

Coverity issue: #59767
tests: Fix copy–paste errors in the OAuth 1 tests

Coverity issues: #59766, #59765
contacts: Remove a redundant xmlFree() call

It was always called on NULL, and was confusing Coverity.

Coverity issue: #59764
tests: Add an extra assertion about pagination

Coverity issue: #59763
core: Disconnect a cancellable in GDataDownloadStream

Ensure to disconnect from the user-provided cancellable when finalising
a GDataDownloadStream.

Coverity issue: #59757