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/gen_opt_test.vim b/src/testdir/gen_opt_test.vim
index 07a744b..1187dd5 100644
--- a/src/testdir/gen_opt_test.vim
+++ b/src/testdir/gen_opt_test.vim
@@ -13,7 +13,7 @@
       \ 'let save_term = &term',
       \ ]
 
-edit option.c
+edit
 /#define p_term
 let end = line('.')
 
@@ -197,7 +197,7 @@
 call add(script, 'let &columns = save_columns')
 call add(script, 'let &lines = save_lines')
 
-call writefile(script, 'testdir/opt_test.vim')
+call writefile(script, 'opt_test.vim')
 
 endif