patch 8.1.0026: terminal test fails with very tall terminal

Problem:    Terminal test fails with very tall terminal. (Tom)
Solution:   Fix the terminal window size in the test.
diff --git a/src/testdir/test_terminal.vim b/src/testdir/test_terminal.vim
index c4e49b7..4cd7757 100644
--- a/src/testdir/test_terminal.vim
+++ b/src/testdir/test_terminal.vim
@@ -270,7 +270,7 @@
 endfunc
 
 func Test_terminal_scrollback()
-  let buf = Run_shell_in_terminal({})
+  let buf = Run_shell_in_terminal({'term_rows': 15})
   set termwinscroll=100
   call writefile(range(150), 'Xtext')
   if has('win32')