gdb/testsuite/

gdb/testsuite/
	* gdb.threads/gcore-thread.exp ($testfile): Match it the .exp
	filename.
	($srcfile): Preserve the original value.
	($testfile): Match it the .exp filename.
	($corefile): New variable.  Substitute it around.
	Use clean_restart.
	($prev_timeout): Remove.
	(load_core): Move core loading into this proc.
	Fix restore of $timeout if load_core fails.