add special cased mentioned in PR review

Author: cre4ture

tests/by-util/test_env.rs

@@ -780,6 +780,17 @@ fn test_env_arg_argv0_overwrite_mixed_with_string_args() {
         .succeeds()
         .stdout_is("aa/bb/cc\n")
         .stderr_is("");
+
+    // three args: normal, string, normal
+    ts.ucmd()
+        .args(&["-a", "sleep"])
+        .args(&["-S-a dirname"])
+        .args(&["-a", "echo"])
+        .arg(&bin)
+        .args(&["aa/bb/cc"])
+        .succeeds()
+        .stdout_is("aa/bb/cc\n")
+        .stderr_is("");
 }
 
 #[cfg(test)]