Generate aggregated coverage report containing all process definitions

[tobiasbehr]

As a developer I want to have a consolidated report (containing all process tests) being generated when I run the tests on my project.

Current behaviour (v1.0.3): When I run multiple test classes, each testing a different process, I only get a report for the first test class.

As a workaround we can use a dedicated instance of ProcessEngineCoverageExtension for each test so that we get one report.html for each.

[rohwerj]

Extracted the issue where test classes sharing an extension do not write the report correctly into #208.
This ticket only tracks the enhancement request to generate an aggregated report.