Add plugin tests

[sinyo-matu]

related issue: #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

fresh-seo/tests/plugin.test.ts

Line 9 in 14b9c05

await withFresh("./tests/fixture/dev.ts", async (address) => {

the WithFresh function will start the Fresh web server of the fixture/ for our test process.

Todo:

• test for sitemap generation.
• test for include functionality
• test for exclude functionality