Fix tests to work in multi-gdb mode.

The Gdb class now can handle connecting to more than one gdb. It
enumerates the harts across all connections, and when asked to select a
hart, it transparently sends future gdb commands to the correct
instance.

Multicore tests still have to be aware of some differences. The main one
is that when executing 'c' in RTOS mode, all harts resume, while in
multi-gdb mode only the current one resumes. Additionally, gdb doesn't
set breakpoints until 'c' is issued, so the hart where breakpoints are
set needs to be resumed before other harts might see them.