Fix multiple-successive-infcall.exp with gdbserver

PR gdb/33791 points out that my recent change to emit the thread ID in
the thread notification message broke multiple-successive-infcall.exp
when run with gdbserver.  This patch fixes the oversight.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33791
Approved-By: Simon Marchi <simon.marchi@efficios.com>