patch 8.0.0429: options test does not always test everything

Problem:    Options test does not always test everything.
Solution:   Fix dependency for opt_test.vim.  Give a message when opt_test.vim
            was not found.
diff --git a/src/testdir/test_options.vim b/src/testdir/test_options.vim
index ac43c1a..9cf5e5f 100644
--- a/src/testdir/test_options.vim
+++ b/src/testdir/test_options.vim
@@ -301,8 +301,9 @@
 endfunc
 
 func Test_set_values()
-  " The file is only generated when running "make test" in the src directory.
   if filereadable('opt_test.vim')
     source opt_test.vim
+  else
+    throw 'Skipped: opt_test.vim does not exist'
   endif
 endfunc