The WatchAddress API includes a flag to indicate if watchpoint should be
for read, modify or both. This API uses 2 booleans, but the 'modify'
flag was ignored and WatchAddress unconditionally watched write
(actually modify).
We now only watch for modify when the modify flag is true.
---
The included test fails prior to this patch and succeeds after. That is
previously specifying `False` for `modify` would still stop on _write_,
but after the patch correctly only stops on _read_
e8d437f827