tests/basics/bytes_add: Add tests for optimised bytes addition.

tests/basics/bytes_add.py

@@ -2,3 +2,7 @@
 
 print(b"123" + b"456")
 print(b"123" + bytearray(2))
+
+print(b"123" + b"") # RHS is empty, can be optimised
+print(b"" + b"123") # LHS is empty, can be optimised
+print(b"" + bytearray(1)) # LHS is empty but can't be optimised