tests(App Help): tests writing of App help message

tests/print_help.rs

@@ -0,0 +1,33 @@
+extern crate clap;
+
+use clap::App;
+
+#[test]
+fn print_app_help() {
+    let mut app = App::new("test")
+        .author("Kevin K.")
+        .about("tests stuff")
+        .version("1.3")
+        .args_from_usage("-f, --flag 'some flag'
+                          --option [opt] 'some option'");
+    // We call a get_matches method to cause --help and --version to be built
+    let _ = app.get_matches_from_safe_borrow(vec![""]);
+
+    // Now we check the output of print_help()
+    let mut help = vec![];
+    app.write_help(&mut help).ok().expect("failed to print help");
+    assert_eq!(&*String::from_utf8_lossy(&*help), &*String::from("test 1.3\n\
+Kevin K.
+tests stuff
+
+USAGE:
+\ttest [FLAGS] [OPTIONS]
+
+FLAGS:
+    -f, --flag       some flag
+    -h, --help       Prints help information
+    -V, --version    Prints version information
+
+OPTIONS:
+        --option <opt>    some option\n"));
+}