fix(terminal): adopt altscreen test for libvterm 0.2 changes

Due to a rewrite of the resize handling logic in libvterm, the behavior
changed. It's actually fixing a bug.

16b8574575

test/functional/terminal/altscreen_spec.lua

@@ -126,13 +126,13 @@ describe(':terminal altscreen', function()
       wait_removal()
       feed('<c-\\><c-n>4k')
       screen:expect([[
-        ^line3                                             |
+        ^                                                  |
                                                           |
                                                           |
         rows: 4, cols: 50                                 |
                                                           |
       ]])
-      eq(8, curbuf('line_count'))
+      eq(9, curbuf('line_count'))
     end)
 
     describe('and after exit', function()
@@ -142,15 +142,11 @@ describe(':terminal altscreen', function()
       end)
 
       it('restore buffer state', function()
-        -- FIXME(tarruda): Note that the last line was lost after restoring the
-        -- screen. This is a libvterm bug: When the main screen is restored it
-        -- seems to "cut" lines that would have been left below the new visible
-        -- screen.
         screen:expect([[
-          line4                                             |
           line5                                             |
           line6                                             |
           line7                                             |
+          line8                                             |
           {3:-- TERMINAL --}                                    |
         ]])
       end)