Fix race condition in the auth db creation

This test deletes a database and then tries to go and write to it
without ensuring that it exists. This is just a race with the auth cache
code trying to recreate its own database. This addition just makes sure
it exists by trying to create it and ignoring any 412 errors if the auth
cache won the race.

apps/couch_httpd/share/www/script/test/reader_acl.js

@@ -18,6 +18,13 @@ couchTests.reader_acl = function(debug) {
   function testFun() {
     try {
       usersDb.deleteDb();
+      try {
+        usersDb.createDb();
+      } catch(e) {
+        if(usersDb.last_req.status != 412) {
+         throw e;
+        }
+      }
       secretDb.deleteDb();
       secretDb.createDb();