tests/basics: Add test for printing OSError when errno is unknown.

tests/basics/errno1.py

@@ -13,3 +13,6 @@ print(type(uerrno.EIO))
 # check that errors are rendered in a nice way
 msg = str(OSError(uerrno.EIO))
 print(msg[:7], msg[-5:])
+
+# check that unknown errno is still rendered
+print(str(OSError(9999)))

tests/basics/errno1.py.exp

@@ -1,2 +1,3 @@
 <class 'int'>
 [Errno  ] EIO
+9999