gdb/testsuite: add tests for Term

While trying to review Andrew's patch here [1], I thought I spotted a
bug in the handling of a CSI, but I had no way to know for sure.  So I
thought it would be useful to have unit tests for the handling of
control characters and control sequences of our toy terminal
implementation.  It might help avoid chasing bugs in the GDB TUI when in
reality it's a problem with the testsuite's terminal implementation.

Add the gdb.tui/tuiterm.exp file to do that.  All currently supported
control sequences and characters are tested, except _csi_m (the one that
handles colors and stuff).  _csi_m should probably be tested too, but it
will require more work.

Fix a few issues that the tests spotted:

 - backspace: according to [3] (table 4-1), a backspace when the cursor
   is at the beginning of a line should have no effect.  Our
   implementation did wrap to the end of the previous line.  Change our
   implementation to match the doc (and the test).
 - insert character: this control sequence is supposed to insert blank
   characters, shifting all the rest of the line right.  The current
   implementation moves N characters right, but it overwrites the
   characters on the right instead of shifting them.  It also doesn't
   insert blank characters at the cursor.
 - Cursor down, forward, next line: off-by-one error when reaching the
   end of the display.
 - erase in display, line: off-by-one errors.
 - vertical line position absolute: allowed setting the cursor outside
   the display, when it should clamp it to the display size.

I found that this web page [2] gave some good clues on the expected
behavior of some control characters or sequences that some other pages
didn't.

[1] https://sourceware.org/pipermail/gdb-patches/2022-March/186433.html
[2] https://docs.microsoft.com/en-us/windows/console/console-virtual-terminal-sequences
[3] https://vt100.net/docs/vt510-rm/chapter4.html#S4.3.3

Change-Id: Iab4141fdcfb7459d1b7c45cc63bd1fcb50a78d5d

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