Tweak TestHarness API and docs (#790) #2457

Job	Run time
cargo clippy (windows-latest)	4m 53s
cargo check (msrv) (wasm32)	14s
cargo check (msrv) (windows-latest)	1m 9s
cargo clippy (macos-latest)	2m 5s
cargo check (aarch64-android)	27s
cargo check (msrv) (macos-latest)	37s
cargo clippy (ubuntu-latest)	3m 18s
cargo clippy (wasm32)	2m 5s
cargo check (msrv) (ubuntu-latest)	37s
cargo doc	42s
formatting	31s
Prime LFS Cache	5s
cargo test (wasm32)	21s
typos	6s
cargo test (windows-latest)	3m 14s
cargo test (macos-latest)	1m 27s
cargo test (ubuntu-latest)	3m 0s
	24m 51s

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