testutils: move global TestBackendData mapping to TestEnvironment #15411

Job	Run time
Build jj-lib without Git support	46s
build (ubuntu-latest)	4m 43s
build (macos-13)	7m 43s
Check protos	33s
Check formatting	17s
build (macos-14)	3m 39s
build (windows-latest)	8m 34s
cargo-deny (advisories)	23s
Clippy check	2m 39s
build (ubuntu-latest, --all-features)	5m 40s
Check that MkDocs can build the docs	31s
cargo-deny (bans licenses sources)	25s
Check that MkDocs can build the docs with Poetry 1.8	18s
	36m 11s

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