[lldb] [test/Register] Add read/write tests for multithreaded process

Add a test to verify that 'register read' and 'register write' commands
work correctly in a multithreaded program, in particular that they read
or write registers for the correct thread.  The tests use locking
to ensure that events are serialized and the test can execute reliably.

Differential Revision: https://reviews.llvm.org/D89248