Factor out the `mdtest` crate #69566

Job	Run time
Determine changes	24s
mkdocs	2m 48s
prek	1m 7s
python package	1m 30s
cargo fmt	20s
cargo build (msrv)	1m 48s
cargo test (wasm)	3m 8s
cargo test (linux)	4m 11s
cargo clippy	2m 1s
cargo test (linux, release)	3m 8s
ty completion evaluation	1m 30s
Fuzz for new ty panics	2m 48s
test ruff-lsp	2m 44s
cargo fuzz build	4m 47s
ecosystem	6m 1s
fuzz parser	7m 52s
benchmarks walltime (build)	2m 27s
test scripts	1m 8s
benchmarks instrumented ty (build)	2m 22s
cargo shear	22s
formatter instabilities and black similarity	3m 5s
cargo test (depot-windows-2022-16)	7m 36s
cargo test (macos-latest)	12m 24s
benchmarks instrumented (ruff)	4m 3s
check playground	-1s
benchmarks instrumented ty (memory: check_file\|micro\|anyio)	4m 47s
benchmarks instrumented ty (memory: attrs\|hydra\|datetype)	3m 28s
benchmarks instrumented ty (simulation: check_file\|micro\|anyio)	6m 25s
benchmarks instrumented ty (simulation: attrs\|hydra\|datetype)	4m 17s
benchmarks walltime (colour_science)	3m 24s
benchmarks walltime (pandas\|tanjun\|altair)	4m 51s
benchmarks walltime (static_frame\|sympy)	4m 54s
benchmarks walltime (pydantic\|multithreaded\|freqtrade)	3m 54s
all required checks passed	4s
	1h 55m 37s

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