Refactor Sliding Sync tests to better utilize the `SlidingSyncBase` (pt. 1) #2421

Job	Run time
Calculate list of debian distros	4s
Build wheels on ubuntu-20.04 for x86_64	1m 39s
Build sdist	0s
Build .deb packages (debian:sid)	8m 45s
Attach assets to release	0s
	10m 28s

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