Initial unit tests and related adjustments

Instead of whitelisting, I would blacklist `./tests/` and include everything else. Otherwise, if we add code to wp-seo.php, it won't be included in the code coverage report.

```xml
<filter>
        <whitelist processUncoveredFilesFromWhitelist="true">
                <directory suffix=".php">./</directory>
                <exclude>
                        <directory suffix=".php">./tests/</directory>
                </exclude>
        </whitelist>
</filter>
```