Test that SIGINT interrupts non-terminating goals on unix.

Cleanup: use tokio::test instead of special test helpers

Added pty_exec.py helper script to run binaries with a pseudoterminal attached (bug won't trigger otherwise).

Tested CI to fail on all unixes (ubuntu+macos) with older rustyline, and pass with new one.

tests-pl/pty_exec.py

@@ -0,0 +1,14 @@
+#!/usr/bin/env python3
+# Pseudoterminal wrapper script
+
+import pty
+from sys import argv
+
+def main():
+    if len(argv) < 2:
+        print(f"Usage: {argv[0]} [command]")
+        return
+    pty.spawn(argv[1:])
+
+if __name__ == "__main__":
+    main()