fix: vortex-layout doc tests require zstd (#6179)

Maybe I am the only one who runs `cargo test -p vortex-layout`? `ci.yml`
uses `--all-features`.

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Signed-off-by: Daniel King <dan@spiraldb.com>

Author: danking

Cargo.lock

@@ -60,6 +60,7 @@ rstest = { workspace = true }
 tokio = { workspace = true, features = ["rt", "macros"] }
 vortex-array = { path = "../vortex-array", features = ["_test-harness"] }
 vortex-io = { path = "../vortex-io", features = ["tokio"] }
+vortex-layout = { path = ".", features = ["zstd"] }
 vortex-utils = { workspace = true, features = ["_test-harness"] }
 
 [features]