cli-test: Add placeholder for executable extension

Help text generated by `clap` contains the full file name (including
extension) of the binary. On Unix-like systems, binaries commonly do not
have any file extension. On Windows, ".exe" is common.

To allow testing such output, introduce a new marker "[EXE]" that is 
substituted accordingly. The idea and syntax is taken from
https://docs.rs/trycmd/1.0.0/trycmd/#toml

crates/radicle-cli-test/src/lib.rs

@@ -481,6 +481,16 @@ impl TestFormula {
                         let assert = OutputAssert::new(output).with_assert(assert.clone());
                         let expected = Self::map_spaced_brackets(&assertion.expected);
 
+                        let expected = {
+                            #[cfg(windows)]
+                            const EXE: &str = ".exe";
+
+                            #[cfg(unix)]
+                            const EXE: &str = "";
+
+                            expected.replace("[EXE]", EXE)
+                        };
+
                         let matches = if test.stderr {
                             assert.stderr_matches(&expected)
                         } else {

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

@@ -9,7 +9,7 @@ Do you have feedback?
  - Mail <feedback@radicle.xyz>
    (Messages are automatically posted to the public #feedback channel on Zulip.)
 
-Usage: rad <COMMAND>
+Usage: rad[EXE] <COMMAND>
 
 Commands:
   auth      Manage identities and profiles