First install Composer. Then you can generate the autoload file and install phpunit with this command line:
composer install
When you do modifications in the source code, be sure that all tests pass:
cd tests
../vendor/bin/phpunit
Tests about docbook can also validate generated docbook content against the official Docbook 5 relaxng schema. If you want to activate validation tests, install xmllint, docbook 5 relaxng, and create an environment variable DOCBOOK_RNG containing "yes" or a path to the relaxng file.
On debian/ubuntu, you just have to install libxml2-utils and docbook5-xml packages, and set DOCBOOK_RNG="yes".