Add test for doctest runtool

Author: ehuss

Diff for: tests/rustdoc/doctest/auxiliary/doctest-runtool.rs

@@ -0,0 +1,21 @@
+// For some reason on Windows, the PATH to the libstd dylib doesn't seem to
+// carry over to running the runtool.
+//@ no-prefer-dynamic
+
+use std::path::Path;
+use std::process::Command;
+
+fn main() {
+    let args: Vec<_> = std::env::args().collect();
+    eprintln!("{args:#?}");
+    assert_eq!(args.len(), 4);
+    assert_eq!(args[1], "arg1");
+    assert_eq!(args[2], "arg2 with space");
+    let path = Path::new(&args[3]);
+    let output = Command::new(path).output().unwrap();
+    // Should fail without env var.
+    assert!(!output.status.success());
+    let output = Command::new(path).env("DOCTEST_RUNTOOL_CHECK", "xyz").output().unwrap();
+    // Should pass with env var.
+    assert!(output.status.success());
+}

Diff for: tests/rustdoc/doctest/doctest-runtool.rs

@@ -0,0 +1,14 @@
+// Tests that the --runtool argument works.
+
+//@ ignore-cross-compile
+//@ aux-bin: doctest-runtool.rs
+//@ compile-flags: --test
+//@ compile-flags: --runtool=auxiliary/bin/doctest-runtool
+//@ compile-flags: --runtool-arg=arg1 --runtool-arg
+//@ compile-flags: 'arg2 with space'
+//@ compile-flags: -Zunstable-options
+
+/// ```
+/// assert_eq!(std::env::var("DOCTEST_RUNTOOL_CHECK"), Ok("xyz".to_string()));
+/// ```
+pub fn main() {}