Add 'pramen-test' artifact that contains fixtures for testing Pramen components

## Background
Currently, fixtures used for testing Pramen sources, sinks, and transformations are in the test code only, and not published as part of an artifact. It would be helpful to have reusable test fixtures in a published artifact.

## Feature
Add 'pramen-test' artifact that contains fixtures for testing Pramen components.

## Proposed Solution
Use 'tests' classifier for the Maven artifact that contains test fixtures.

Reference: https://www.baeldung.com/maven-artifact-classifiers#2-consuming-tests-jar