Two fixes for test suite's terminal

Two fixes for test suite's terminal

Exactly which escape sequences are emitted by gdb in TUI mode are
determined largely by the curses implementation.  Testing my latest
(as yet unsubmitted) series to refactor the TUI showed a couple of
failures that I tracked to the test suite's terminal implementation.

In particular, the CSI "@" sequence was not implemented; and the CSI
"X" sequence was implemented incorrectly.

This patch fixes both of these problems.  Tested on x86-64 Fedora 28.

gdb/testsuite/ChangeLog
2019-07-29  Tom Tromey  <tom@tromey.com>

	* lib/tuiterm.exp (Term::_csi_@): New proc.
	(Term::_csi_X): Don't move cursor.