patch 9.1.0808: Terminal scrollback doesn't shrink when decreasing 'termwinscroll'

Problem:  Terminal scrollback doesn't shrink when reducing
          'termwinscroll'
Solution: Check if option value was decreased (Milly).

closes: #15904

Signed-off-by: Milly <milly.ca@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/gen_opt_test.vim b/src/testdir/gen_opt_test.vim
index 1c231f9..34fffef 100644
--- a/src/testdir/gen_opt_test.vim
+++ b/src/testdir/gen_opt_test.vim
@@ -103,6 +103,7 @@
       \ 'sidescroll': [[0, 1, 8, 999], [-1]],
       \ 'sidescrolloff': [[0, 1, 8, 999], [-1]],
       \ 'tabstop': [[1, 4, 8, 12, 9999], [-1, 0, 10000]],
+      \ 'termwinscroll': [[1, 100, 99999], [-1, 0]],
       \ 'textwidth': [[0, 1, 8, 99], [-1]],
       \ 'timeoutlen': [[0, 8, 99999], [-1]],
       \ 'titlelen': [[0, 1, 8, 9999], [-1]],