gtk_text_iter_backward_line() checks the value of
real->line_char_offset without previously calling
ensure_char_offsets (real) to make sure the former
is up-to-date.

As a consequence of this, when gtk_text_iter_backward_line()
is called after a gtk_text_buffer_insert_range() in the
first line of buffer, the iter is not moved to the start of
the line, and the return value is wrong.

Fixed by adding the ensure_char_offsets() call.

A test case for this bug is added to the textiter gtk testsuite.