test: Include unstable-ext

Author: epage

Makefile

@@ -16,7 +16,7 @@ _FEATURES = minimal default wasm full debug release
 _FEATURES_minimal = --no-default-features --features "std"
 _FEATURES_default =
 _FEATURES_wasm = --no-default-features --features "std help usage error-context suggestions" --features "deprecated derive cargo env unicode string"
-_FEATURES_full = --features "deprecated derive cargo env unicode string wrap_help"
+_FEATURES_full = --features "deprecated derive cargo env unicode string wrap_help unstable-ext"
 _FEATURES_next = ${_FEATURES_full} --features unstable-v5
 _FEATURES_debug = ${_FEATURES_full} --features debug --features clap_complete/debug
 _FEATURES_release = ${_FEATURES_full} --release