patch 9.0.0543: insufficient testing for assert and test functions

Problem:    Insufficient testing for assert and test functions.
Solution:   Add a few more tests. (Yegappan Lakshmanan, closes #11190)
diff --git a/src/testdir/test_options.vim b/src/testdir/test_options.vim
index ec11494..a0ee53b 100644
--- a/src/testdir/test_options.vim
+++ b/src/testdir/test_options.vim
@@ -815,6 +815,7 @@
   call assert_match('file2', execute('bn', ''))
   bwipe
   bwipe
+  call assert_fails('call test_getvalue("abc")', 'E475:')
 endfunc
 
 func Test_local_scrolloff()