image_viewer: Fix cargo test errors (#24404)

This PR fixes the errors when running `cargo test` in the `image_viewer`
crate.

Release Notes:

- N/A

crates/image_viewer/Cargo.toml

@@ -12,6 +12,9 @@ workspace = true
 path = "src/image_viewer.rs"
 doctest = false
 
+[features]
+test-support = ["gpui/test-support"]
+
 [dependencies]
 anyhow.workspace = true
 db.workspace = true
@@ -25,5 +28,5 @@ ui.workspace = true
 util.workspace = true
 workspace.workspace = true
 
-[features]
-test-support = ["gpui/test-support"]
+[dev-dependencies]
+editor = { workspace = true, features = ["test-support"] }