testsuite/gdb.trace: Fix expected message on continue.

testsuite/gdb.trace: Fix expected message on continue.

This is fallout from f303dbd60d9c7984832446eeb9d4e4d89703c615.

The testcases themselves are single-threaded, but they load the IPA library,
which injects a thread in the inferior - making them multithreaded.
This results in printing the thread numbers in breakpoint messages.

gdb/testsuite/ChangeLog:

	* gdb.trace/ftrace.exp: Fix expected message on continue.
	* gdb.trace/pending.exp: Fix expected message on continue.
	* gdb.trace/trace-break.exp: Fix expected message on continue.