Resolve "gio/gdbus-threading test sometimes fails in CI"

Merged Philip Withnall requested to merge pwithnall/glib:1515-gdbus-threading into master

See the individual commit messages for the details of this varied, sometimes elated and sometimes tragic journey.

With this branch, gdbus-threading passes for me 1000 times in a row (ignoring failures due to an unrelated GCancellable race, #1841 (closed)).

Closes #1515 (closed)

Edited by Philip Withnall

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