Fix cargo shear warnings (#64340)

Also updates CI to always run this - that way we ensure that disabling
or enabling tests does not become stale at some point and thus can
safely add this.

Release Notes:

- N/A
- [GPUI] N/A

crates/gpui_platform/Cargo.toml

@@ -10,6 +10,7 @@ workspace = true
 
 [lib]
 path = "src/gpui_platform.rs"
+doctest = false
 
 [features]
 default = []