Fix internal warning when "gdb -p xxx"

The issue that was fixed by b4ab256ded5020a82ff7ce8dc485e7882fc5b6a7
can not be found in regression test.
Update attach.exp to test it.

https://sourceware.org/ml/gdb-patches/2014-03/msg00438.html

2014-03-21  Hui Zhu  <hui@codesourcery.com>

	* gdb.base/attach.exp (do_command_attach_tests): New.