build

lib: merge `testutils` crate directly into `jj_lib::testutils` #12239

Job	Run time
Check protos	23s
Build jj-lib without Git support	43s
cargo-deny (advisories)	24s
cargo-deny (bans licenses sources)	21s
build (ubuntu-latest)	3m 58s
Clippy check	2m 24s
Check that MkDocs can build the docs	23s
Check that MkDocs can build the docs with Poetry 1.8	20s
build (macos-14)	3m 6s
Check formatting	18s
build (windows-latest)	9m 11s
build (ubuntu-latest, --all-features)	5m 6s
	26m 37s

Provide feedback