[lldb] Require paused process and frame for "register info" command (#67124)

Prior to this the command would simply crash when run on a running
process.

Of the three register commands, "info" was the only one missing these
requirements. On some level it makes sense because you're not going to
read a value or modify anything, but practically I think lldb assumes
any time you're going to access register related stuff, the process
should be paused.

I noticed this debugging with a remote gdb stub, so I've recreated that
scenario using attach in a new test case.