Add plugin tests

[sinyo-matu]

related issue: https://github.com/xstevenyung/fresh-seo/issues/25

Based on the Fresh Project, I wrote some code for plugin testing.

In the tests/ directory, the fixture/ directory is a stub Fresh site as a test target. In the file plugin.test.ts https://github.com/xstevenyung/fresh-seo/blob/14b9c057535a47fa41a64e8f8e52c7fe3d2312fe/tests/plugin.test.ts#L9 the WithFresh function will start the Fresh web server of the fixture/ for our test process.

Todo:

• [x] test for sitemap generation.
• [ ] test for include functionality
• [ ] test for exclude functionality