Test: add integration + e2e tests

[erezrokah]

Related to https://github.com/netlify-labs/netlify-plugin-sitemap/issues/71

This is not the first time we've had a regression with sites deploying from the root directory. The plugin is meant to deal with both absolute and relative paths for publish directory and excludes.

We should add some tests, preferably on real sites, for these different use cases (at the moment we only have unit tests for few cases).