Splits register_commands into a passing test on Linux (to improve test coverage), 
and a failing test for register expressions.

Also factors out common setup code in preparation to add a few new tests.
Reviewed by: Daniel Malea


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@180109 91177308-0d34-0410-b5e6-96231b3b80d8