test: Add tests for const fn

src/error.rs

@@ -174,6 +174,12 @@ mod tests {
         assert_eq!(TryFromExitStatusError::new(None).code(), None);
     }
 
+    #[cfg(feature = "std")]
+    #[test]
+    const fn code_try_from_exit_status_error_is_const_fn() {
+        const _: Option<i32> = TryFromExitStatusError::new(None).code();
+    }
+
     #[cfg(feature = "std")]
     #[test]
     fn display_try_from_exit_status_error() {

src/exit_code.rs

@@ -609,6 +609,11 @@ mod tests {
         assert!(!ExitCode::Config.is_success());
     }
 
+    #[test]
+    const fn is_success_is_const_fn() {
+        const _: bool = ExitCode::Ok.is_success();
+    }
+
     #[test]
     fn is_failure_for_successful_termination() {
         assert!(!ExitCode::Ok.is_failure());
@@ -633,6 +638,11 @@ mod tests {
         assert!(ExitCode::Config.is_failure());
     }
 
+    #[test]
+    const fn is_failure_is_const_fn() {
+        const _: bool = ExitCode::Ok.is_failure();
+    }
+
     #[cfg(feature = "std")]
     #[test]
     fn source() {