Improve API

- method to automatically find and run all tests in a specified directory
- TestLogger implementations: NULL and PrintStreamLogger
- functions to count the failures/errors in test results
Add runtime dependencies to the xproc-engine-daisy-pipeline POM

so that these dependencies are automatically included in the test-phase
classpath for projects that have xproc-engine-daisy-pipeline as a test
scope dependency (typically for XProcSpec testing)