test(old): fix test_tabline failure (#32907)

Also fix tests after test_sha256 in test_alot.vim being skipped.

test/old/testdir/test_sha256.vim

@@ -1,7 +1,7 @@
 " Tests for the sha256() function.
 
 source check.vim
-CheckFeature cryptv
+" CheckFeature cryptv
 CheckFunction sha256
 
 function Test_sha256()

test/old/testdir/test_tabline.vim

@@ -210,6 +210,9 @@ endfunc
 
 " Test 'tabline' with truncated double-width label at the start.
 func Test_tabline_truncated_double_width()
+  let save_TabLine = nvim_get_hl(0, #{name: 'TabLine'})
+  " Nvim: avoid combining TabLine with TabLineFill in custom tabline
+  hi TabLine cterm=underline,nocombine gui=underline,nocombine
   tabnew
   redraw
   call assert_match('X$', Screenline(1))
@@ -226,6 +229,7 @@ func Test_tabline_truncated_double_width()
 
   bw!
   set tabline=
+  call nvim_set_hl(0, 'TabLine', save_TabLine)
 endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab