patch 8.2.4828: fix for unmapping simplified key not fully tested

Problem:    Fix for unmapping simplified key not fully tested.
Solution:   Add a test case. (closes #10292)
diff --git a/src/testdir/test_mapping.vim b/src/testdir/test_mapping.vim
index e0008d6..b413f4f 100644
--- a/src/testdir/test_mapping.vim
+++ b/src/testdir/test_mapping.vim
@@ -1627,7 +1627,7 @@
   set mouse&
 endfunc
 
-func Test_unmap_simplfied()
+func Test_unmap_simplifiable()
   map <C-I> foo
   map <Tab> bar
   call assert_equal('foo', maparg('<C-I>'))
@@ -1636,6 +1636,11 @@
   call assert_equal('', maparg('<C-I>'))
   call assert_equal('bar', maparg('<Tab>'))
   unmap <Tab>
+
+  map <C-I> foo
+  unmap <Tab>
+  " This should not error
+  unmap <C-I>
 endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab