gdb.threads/manythreads.exp: can't read "test": no such variable

gdb.threads/manythreads.exp: can't read "test": no such variable

If interrupt_and_wait manages to trigger the FAIL path, we get:

  ERROR OCCURED: can't read "test": no such variable

gdb/testsuite/ChangeLog:
2015-04-01  Pedro Alves  <palves@redhat.com>

	* gdb.threads/manythreads.exp (interrupt_and_wait): Pass $message
	to fail instead of non-existent $test.