Site tests passes even with warnings

[nicholaschuayunzhi]

Tell us about your environment

• MarkBind Version:
  1.16.1

What did you do? Please include the actual source code causing the issue.
Tests on Travis passes even though are missing files.

What did you expect to happen?
Travis should fail.

What actually happened? Please include the actual, raw output.

  __  __                  _      ____    _               _ 
 |  \/  |   __ _   _ __  | | __ | __ )  (_)  _ __     __| |
 | |\/| |  / _` | | '__| | |/ / |  _ \  | | | '_ \   / _` |
 | |  | | | (_| | | |    |   <  | |_) | | | | | | | | (_| |
 |_|  |_|  \__,_| |_|    |_|\_\ |____/  |_| |_| |_|  \__,_|
                                                           
 v1.16.1
info: Website generation started at 05:34:38
warning: ENOENT: no such file or directory, open '/home/travis/build/MarkBind/markbind/test/test_site/sub_site/_markbind/variables.md'
info: Building assets...
info: Assets built
info: Generating pages...
info: Pages built
info: Website generation complete! Total build time: 0.645s
info: Build success!
info: Site data built
Test result: PASSED
The command "npm test" exited with 0.

Why
Markbind detects missing file but only returns a warning.
Committed expected output is same as built site (perhaps dev error or this warning does not affect final site).

For discussion

1. How to resolve this error
   • fix test case
2. How can we prevent this
   • MarkBind test should fail even with warnings
   • Ensure that expected site is checked

[acjh]

Related: #581