tests/heapalloc_str: Test no-replacement case for str.replace().

tests/micropython/heapalloc_str.py

@@ -3,6 +3,7 @@ import micropython
 
 micropython.heap_lock()
 
+# Concatenating empty string returns original string
 b"" + b""
 b"" + b"1"
 b"2" + b""
@@ -11,4 +12,7 @@ b"2" + b""
 "" + "1"
 "2" + ""
 
+# If no replacements done, returns original string
+"foo".replace(",", "_")
+
 micropython.heap_unlock()