cli/test: Output of `rad --help`

With migration subcommands to `clap` planned one-by-one,
prepare this to catch regressions.

crates/radicle-cli/examples/rad-help.md

@@ -0,0 +1,42 @@
+```
+$ rad --help
+rad [..]
+Radicle command line interface
+
+Usage: rad <command> [--help]
+Common `rad` commands used in various situations:
+
+	auth         Manage identities and profiles
+	block        Block repositories or nodes from being seeded or followed
+	checkout     Checkout a repository into the local directory
+	clone        Clone a Radicle repository
+	config       Manage your local Radicle configuration
+	fork         Create a fork of a repository
+	help         CLI help
+	id           Manage repository identities
+	init         Initialize a Radicle repository
+	inbox        Manage your Radicle notifications
+	inspect      Inspect a Radicle repository
+	issue        Manage issues
+	ls           List repositories
+	node         Control and query the Radicle Node
+	patch        Manage patches
+	path         Display the Radicle home path
+	clean        Remove all remotes from a repository
+	self         Show information about your identity and device
+	seed         Manage repository seeding policies
+	follow       Manage node follow policies
+	unblock      Unblock repositories or nodes to allow them to be seeded or followed
+	unfollow     Unfollow a peer
+	unseed       Remove repository seeding policies
+	remote       Manage a repository's remotes
+	stats        Displays aggregated repository and node metrics
+	sync         Sync repositories to the network
+
+See `rad <command> --help` to learn about a specific command.
+
+Do you have feedback?
+ - Chat[..]
+ - Mail[..]
+   (Messages are automatically posted to the public #feedback channel on Zulip.)
+```

crates/radicle-cli/tests/commands.rs

@@ -86,6 +86,11 @@ fn program_reports_version(program: &str) -> bool {
     }
 }
 
+#[test]
+fn rad_help() {
+    Environment::alice(["rad-help"]);
+}
+
 #[test]
 fn rad_auth() {
     test("examples/rad-auth.md", Path::new("."), None, []).unwrap();