os: Add fork()/pipe() test.

os/test_fork.py

@@ -0,0 +1,17 @@
+import os
+
+r, w = os.pipe()
+print(r, w)
+
+pid = os.fork()
+print("hello", pid)
+
+if not pid:
+    print("pid:", os.getpid())
+    os.close(r)
+    os.write(w, "from child")
+    os._exit(0)
+else:
+    os.close(w)
+    print(os.read(r, 100))
+    print(os.waitpid(pid, 0))