Add tests

[busykoala]

Tested should be if:

• Email are sent to expected address and have an attachment.
• An excel workbook can be generated from given data resulting in the same file as an examplar file.
• All links are found in a PloneSite and broken one can be extracted.
• All broken relations are found in a Plone Site.

[busykoala]

To test the linkchecker we need to have a setup similar to that:

We need these things due to the need to have different settings (e.g. recipient, timeout) for different plone sites and need relations/links on these sites in IRichText fields.

Also it might make sense to test different types of broken links (different status code and errors). The challenge there is that they should be in an isolated environment.