Prevent convenience registers from being included in "read register" as they are derived registers.

- Also refactors TestRegisters.py because test_convenience_registers_with_process_attach now fails with an assert.

TODO: Cross-reference the skipOnLinux decorator with a bugzilla report after root-causing this issue.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@181737 91177308-0d34-0410-b5e6-96231b3b80d8