Update and extend unit tests for PdfBox 1.8.6

The current version of PdfBox (1.8.6) now passes all unit tests. In addition `domFromPdfX` are now being tested.
Older releases (such as 1.6.0 and 1.7.1) do not pass all test cases as newer versions are able to detect bold text. The interface itself does work as intended tough.

In addition I changed `pdfbox-jar` to `PDFBOX_JAR`: This way you can invoke `PDFBOX_JAR=yourpdfbox.jar phpunit -v` which eases the unit testing with different versions of PdfBox.