Add a basic test for 'memory region'

Summary:
The 'memory region' command is at the moment not tested at all by our test suite.
This patch just adds a basic test that at least provides some basic testing.

Reviewers: #lldb, davide

Reviewed By: #lldb, davide

Subscribers: vsk, davide, lldb-commits

Differential Revision: https://reviews.llvm.org/D51930

llvm-svn: 342042