Add regression test for rust-lang#122722

Author: GuillaumeGomez

tests/run-make/rustdoc-test-args/foo.rs

@@ -0,0 +1,3 @@
+//! ```
+//! let x = 12;
+//! ```

tests/run-make/rustdoc-test-args/rmake.rs

@@ -0,0 +1,18 @@
+extern crate run_make_support;
+
+use run_make_support::{out_dir, rustdoc};
+use std::{fs, iter};
+use std::path::Path;
+
+fn generate_a_lot_of_cfgs(path: &Path) {
+    let content = iter::repeat("--cfg=a\n").take(100_000).collect::<String>();
+    fs::write(path, content.as_bytes()).expect("failed to create args file");
+}
+
+fn main() {
+    let arg_file = out_dir().join("args");
+    generate_a_lot_of_cfgs(&arg_file);
+
+    let arg_file = format!("@{}", arg_file.display());
+    rustdoc().arg("--test").arg(&arg_file).arg("foo.rs").run();
+}