tests/heapalloc_str: Test for alloc-free string operations.

Starts with concatenation with an empty string.

tests/micropython/heapalloc_str.py

+# String operations which don't require allocation
+import micropython
+
+micropython.heap_lock()
+
+b"" + b""
+b"" + b"1"
+b"2" + b""
+
+"" + ""
+"" + "1"
+"2" + ""
+
+micropython.heap_unlock()