gdb/testing/tui: add new _csi_{L,S,T}

This patch was original part of this series:

  https://sourceware.org/pipermail/gdb-patches/2022-March/186429.html
  https://sourceware.org/pipermail/gdb-patches/2022-March/186433.html

I've pulled this out as it might be useful ahead of the bigger series
being merged.

This commit adds:

  _csi_L - insert line
  _csi_S - pan down
  _csi_T - pan up