tests: fix flaky 'scrollback' option deletes lines (only) if necessary (#11003)

Using `screen:expect` with the complete grid appears to fix its
flakiness.

Fixes https://github.com/neovim/neovim/issues/10723.

test/functional/terminal/scrollback_spec.lua

@@ -438,9 +438,29 @@ describe("'scrollback' option", function()
     command('sleep 100m')
 
     feed_data(nvim_dir.."/shell-test REP 41 line"..(iswin() and '\r' or '\n'))
-    screen:expect{any='40: line                      '}
+    if iswin() then
+      screen:expect{grid=[[
+        37: line                      |
+        38: line                      |
+        39: line                      |
+        40: line                      |
+                                      |
+        ${1: }                            |
+        {3:-- TERMINAL --}                |
+      ]]}
+    else
+      screen:expect{grid=[[
+        36: line                      |
+        37: line                      |
+        38: line                      |
+        39: line                      |
+        40: line                      |
+        {IGNORE}|
+        {3:-- TERMINAL --}                |
+      ]]}
+    end
+    expect_lines(58)
 
-    retry(nil, nil, function() expect_lines(58) end)
     -- Verify off-screen state
     eq((iswin() and '36: line' or '35: line'), eval("getline(line('w0') - 1)"))
     eq((iswin() and '27: line' or '26: line'), eval("getline(line('w0') - 10)"))