test(derive): Require opt-in

These are now running in other situations in CI and feature combinations
have odd side effects on the output.

Cargo.toml

@@ -170,6 +170,7 @@ string = ["clap_builder/string"]  # Allow runtime generated strings
 unstable-v5 = ["clap_builder/unstable-v5", "clap_derive?/unstable-v5", "deprecated"]
 unstable-ext = ["clap_builder/unstable-ext"]
 unstable-styles = ["clap_builder/unstable-styles"]  # deprecated
+unstable-derive-ui-tests = []
 
 [lib]
 bench = false

Makefile

@@ -37,7 +37,7 @@ clippy-%:
 	cargo clippy ${_FEATURES_${@:clippy-%=%}} ${ARGS} --all-targets -- -D warnings -A deprecated
 
 test-ui-%:
-	cargo +${STABLE} test --test derive_ui --features derive ${_FEATURES_${@:test-ui-%=%}}
+	cargo +${STABLE} test --test derive_ui --features derive,unstable-derive-ui-tests ${_FEATURES_${@:test-ui-%=%}}
 
 doc:
 	cargo doc --workspace --all-features --no-deps --document-private-items

tests/derive_ui.rs

@@ -5,6 +5,8 @@
 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
 // option. This file may not be copied, modified, or distributed
 
+#![cfg(feature = "unstable-derive-ui-tests")]
+
 #[cfg(feature = "derive")]
 #[rustversion::attr(not(stable(1.80)), ignore)] // STABLE
 #[test]