uefi-test-runner: Add clarifying comment for nonesting shell variable

Co-authored-by: Philipp Schuster <[email protected]>

Author: RenTrieu

uefi-test-runner/src/proto/shell.rs

@@ -10,6 +10,7 @@ pub fn test_env(shell: &ScopedProtocol<Shell>) {
     /* Test retrieving list of environment variable names */
     let mut cur_env_vec = shell.get_envs();
     assert_eq!(cur_env_vec.next().unwrap(), cstr16!("path"),);
+    // check pre-defined shell variables; see UEFI Shell spec
     assert_eq!(cur_env_vec.next().unwrap(), cstr16!("nonesting"),);
     let cur_env_vec = shell.get_envs();
     let default_len = cur_env_vec.count();