Please describe the problem you are trying to solve.

Exporter tests only test if they don't fail while exporting.
This still leaves many open questions:

• do all expected files exist?
• have empty piece files been correctly skipped?
• is their content correct?

Describe the solution you propose.

The first two could be as simple as deleting files before the test and checking for existing files afterwards.

The final one is more tricky. Checking the syntax of the XML-based formats should be easy, as preCICE already relies on libxml2.
Checking the semantics could be done using a XSD schema file, but VTK doesn't seem to provide one.
Checking the content should be easy for XML-based files, but will be hacky for legacy VTK.
Other files like the CSV and the series (JSON) files require additional dependencies, which may not be worth the hassle.

Additional context

Follow-up of #2055