Add a test that stdin is always handed-off as blocking

No functional change here, just closing a gap in our coverage.

tests/pexpects/stdin_nonblocking.py

@@ -0,0 +1,39 @@
+#!/usr/bin/env python3
+# Verify that stdin is properly set to blocking even if a job tweaks it.
+from pexpect_helper import SpawnedProc
+import sys
+import signal
+import time
+import os
+
+sp = SpawnedProc()
+send, sendline, expect_prompt, expect_str, sleep = (
+    sp.send,
+    sp.sendline,
+    sp.expect_prompt,
+    sp.expect_str,
+    sp.sleep,
+)
+
+# Launch fish_test_helper.
+expect_prompt()
+exe_path = os.environ.get("fish_test_helper")
+sendline(exe_path + " stdin_make_nonblocking")
+
+expect_str("stdin was blocking")
+sleep(0.1)
+
+send("\x1A")  # ctrl-Z
+expect_prompt("has stopped")
+
+# We don't "restore" non-blocking state when continuing a stopped job.
+sleep(0.1)
+sendline("fg")
+expect_str("stdin was blocking")
+
+# Kill the job and do it again.
+send("\x03")  # ctrl-c
+expect_prompt("")
+sendline(exe_path + " stdin_make_nonblocking")
+expect_str("stdin was blocking")
+send("\x03")  # ctrl-c