patch 8.2.0418: code in eval.c not sufficiently covered by tests

Problem:    Code in eval.c not sufficiently covered by tests.
Solution:   Add more tests. (Yegappan Lakshmanan, closes #5815)
diff --git a/src/testdir/test_unlet.vim b/src/testdir/test_unlet.vim
index 81c20df..dad0953 100644
--- a/src/testdir/test_unlet.vim
+++ b/src/testdir/test_unlet.vim
@@ -21,6 +21,12 @@
 func Test_unlet_fails()
   call assert_fails('unlet v:["count"]', 'E46:')
   call assert_fails('unlet $', 'E475:')
+  let v = {}
+  call assert_fails('unlet v[:]', 'E719:')
+  let l = []
+  call assert_fails("unlet l['k'", 'E111:')
+  let d = {'k' : 1}
+  call assert_fails("unlet d.k2", 'E716:')
 endfunc
 
 func Test_unlet_env()