Added tests for q{f,s}ThreadInfo for attached processes.

Added helper methods:
prep_debug_monitor_and_inferior(): properly handles
the two cases of running the stub against an inferior process
launched by the stub, and one attached to by the stub.  See
docs for function: simplifies test creation for tests that want
to test the same operations against a launched and attached inferior.

Added the q{f,s}ThreadInfo and $qC response comparison test (verifies
they both return the same thing) when the process is attached rather
than launched by the stub.

Modified the previous two tests added to make use of the new
prep_debug_monitor_and_inferior() facility.