functests: Add some more tests

test/functional/eval/changedtick_spec.lua

@@ -119,4 +119,26 @@ describe('b:changedtick', function()
     feed(':echo b:<Tab><Home>let cmdline="<End>"<CR>')
     eq('echo b:changedtick', meths.get_var('cmdline'))
   end)
+  it('cannot be changed by filter() or map()', function()
+    eq(2, changedtick())
+    eq('\nE795: Cannot delete variable filter() argument',
+       redir_exec('call filter(b:, 0)'))
+    eq('\nE742: Cannot change value of map() argument',
+       redir_exec('call map(b:, 0)'))
+    eq('\nE742: Cannot change value of map() argument',
+       redir_exec('call map(b:, "v:val")'))
+    eq(2, changedtick())
+  end)
+  it('cannot be remove()d', function()
+    eq(2, changedtick())
+    eq('\nE795: Cannot delete variable remove() argument',
+       redir_exec('call remove(b:, "changedtick")'))
+    eq(2, changedtick())
+  end)
+  it('does not inherit VAR_FIXED when copying dictionary over', function()
+    eq(2, changedtick())
+    eq('', redir_exec('let d1 = copy(b:)|let d1.changedtick = 42'))
+    eq('', redir_exec('let d2 = copy(b:)|unlet d2.changedtick'))
+    eq(2, changedtick())
+  end)
 end)