vim-patch:8.1.1543: const test fails with small features

Problem:    Const test fails with small features.
Solution:   Don't unlet non-existing variables.
b6e3b88ec8

src/nvim/testdir/test_const.vim

@@ -38,8 +38,10 @@ func Test_define_var_with_lock()
     unlet F
     unlet l
     unlet d
+    if has('channel')
         unlet j
         unlet c
+    endif
     unlet b
     unlet n
 endfunc