This makes it easier to test the placeholder replacements

Adds tests for templates containing none or any of the available
placeholders and verifies that they are correctly used to replace values
in the way it works currently.

Note, that currently it fails to replace multiple of the same
placeholders.

A pull author refers to the username, not the handle of the author. So
it does not contain the `@` character. We can always add this later. For
reference, that could like this:

```
`pull_author_handle` | The handle of the original pull request's author,
e.g. `@korthout`
```