Fix the crashlog.py script's use of the load_address property.

This property is explicitly for use only in the interactive editor,
and NOT in commands.  It's use worked until we got more careful about
not leaving lldb.target lying around in the script interpreter.

I also added a quick sniff test for the save_crashlog command.

<rdar://problem/60350620>
Differential Revision: https://reviews.llvm.org/D80680