build

Use the new test backend more in tests, fix another `MergedTree` bug #6958

Job	Run time
cargo-deny (advisories)	27s
build (ubuntu-latest)	8m 39s
cargo-deny (bans licenses sources)	31s
Check protos	1m 28s
build (macos-13)	4m 4s
build (windows-latest)	11m 22s
Clippy check	3m 2s
build (ubuntu-latest, --all-features)	6m 30s
Check that MkDocs can build the docs	25s
Check formatting	19s
	36m 47s

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