fix some simple thinkos in the test suite

fix some simple thinkos in the test suite

This fixes some parallelization thinkos from a while ago.  I'm not
sure how the problems ever slipped through.  In addition to a thinko
fix in twice.exp, this also finishes fixing it up for parallelization.

2013-11-04  Tom Tromey  <tromey@redhat.com>

	* gdb.base/gcore-buffer-overflow.exp: Use
	standard_output_file, not standard_testfile.
	* gdb.base/twice.exp: Use standard_testfile, not
	standard_output_file.  Use gdb_remote_download.