Add new phpunit-autopkgtest binary for autodep8 runs

This new script will be called by autodep8 to generate DEP8 tests for phpunit runs.

This also introduces a minor change in phpabtpl to allow --require-file to receive an absolute path. This way we can create templates by passing installed autoloaders.

On a local test run, I was able to get a 40% success rate when using this script to generate successful test runs for phpunit reverse build dependencies (the universe of packages used included the ones which do not have test suites - these were considered failures).