build

Use the new test backend more in tests, fix another `MergedTree` bug #6959

Job	Run time
Check protos	39s
cargo-deny (advisories)	27s
Clippy check	3m 18s
cargo-deny (bans licenses sources)	33s
build (ubuntu-latest)	9m 7s
Check that MkDocs can build the docs	28s
Check formatting	18s
build (macos-13)	5m 39s
build (windows-latest)	13m 28s
build (ubuntu-latest, --all-features)	7m 22s
	41m 19s

Provide feedback