gdb/testsuite/tui: implement _csi_P proc

Since commit 3cd522938792 ("Change the pager to a ui_file"), I see these
errors when running gdb.tui/scroll.exp:

    ERROR: invalid command name "_csi_P"
        while executing
    "::gdb_tcl_unknown _csi_P 2"
        ("uplevel" body line 1)
        invoked from within
    "uplevel 1 ::gdb_tcl_unknown $args"
        (procedure "::unknown" line 5)
        invoked from within
    "_csi_P 2"
        ("eval" body line 1)
        invoked from within
    "eval _csi_$cmd $params"

It looks like GDB is emitting a CSI that it did not emit before, the
"Delete character" one:

  https://vt100.net/docs/vt510-rm/DCH.html

Implement it.

Co-Authored-By: Andrew Burgess <aburgess@redhat.com>
Change-Id: I5bf86b6104d51b0623a26a69df83d1ca9a4851b7

gdb/testsuite/gdb.tui/tuiterm.exp

@@ -492,6 +492,34 @@ proc test_delete_line { } {
     } 3 0
 }
 
+proc test_delete_character { } {
+    Term::_move_cursor 2 1
+
+    Term::_csi_P
+    check "delete character, default param" {
+	"abcdefgh"
+	"ijlmnop "
+	"qrstuvwx"
+	"yz01234 "
+    } 2 1
+
+    Term::_csi_P 3
+    check "delete character, explicit param" {
+	"abcdefgh"
+	"ijop    "
+	"qrstuvwx"
+	"yz01234 "
+    } 2 1
+
+    Term::_csi_P 12
+    check "delete character, more than number of columns" {
+	"abcdefgh"
+	"ij      "
+	"qrstuvwx"
+	"yz01234 "
+    } 2 1
+}
+
 proc test_erase_character { } {
     Term::_move_cursor 3 2
     Term::_csi_X
@@ -600,6 +628,7 @@ foreach_with_prefix test {
     test_erase_in_display
     test_erase_in_line
     test_delete_line
+    test_delete_character
     test_erase_character
     test_repeat
     test_vertical_line_position_absolute

gdb/testsuite/lib/tuiterm.exp

@@ -349,6 +349,33 @@ namespace eval Term {
 	}
     }
 
+    # Delete Character.
+    #
+    # https://vt100.net/docs/vt510-rm/DCH.html
+    proc _csi_P {args} {
+	set count [_default [lindex $args 0] 1]
+
+	_log_cur "Delete character ($count)" {
+	    variable _cur_row
+	    variable _cur_col
+	    variable _chars
+	    variable _cols
+
+	    # Move all characters right of the cursor N positions left.
+	    set out_col [expr $_cur_col]
+	    set in_col [expr $_cur_col + $count]
+
+	    while {$in_col < $_cols} {
+		set _chars($out_col,$_cur_row) $_chars($in_col,$_cur_row)
+		incr in_col
+		incr out_col
+	    }
+
+	    # Clear the rest of the line.
+	    _clear_in_line $out_col $_cols $_cur_row
+	}
+    }
+
     # Erase chars.
     #
     # https://vt100.net/docs/vt510-rm/ECH.html