Test "set width/height -1"

As a follow up to the previous commit, add a test for "set
width/height -1", to make sure we don't overflow in readline with
negative values either.

gdb/testsuite/ChangeLog:
2019-02-27  Pedro Alves  <palves@redhat.com>

	* gdb.base/page.exp: Add tests for "set width/height -1".