gdb, testsuite: unify 'Switching to thread' message

The 'normal_stop ()' function in infrun prints

  [Switching to Thread 0x155554dff6c0 (LWP 77976)]

on thread switch.  The 'print_selected_thread_frame ()' function
in thread.c prints

  [Switching to thread 2 (Thread 0x155554dff6c0 (LWP 96643))]

Ensure consistent output by updating the thread switch message in
'normal_stop'.

Approved-By: Tom Tromey <tom@tromey.com>