gdb/testsuite/tui: implement _csi_P proc

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