Cobertura updates/adherence

Currently SQLCover doesn't populate the `filename` field in its Cobertura report. Per the [DTD](https://github.com/cobertura/cobertura/blob/master/cobertura/src/site/htdocs/xml/coverage-03.dtd), this is a required field.

Since many downstream tools that consume Cobertura will assume this field has a value, I think it is better to put *something* rather than nothing, to make a stronger attempt to adhere to the format and increase output ingestion compatibility.

There is also a [0.4 DTD](https://github.com/cobertura/cobertura/tree/master/cobertura/src/site/htdocs/xml) available vs the 0.3 in use currently.

Compare SQLCover's Cobertura output with the referenced cobertura/src/site/htdocs/xml/coverage-03.dtd and the available 0.4 DTD. Locate the Cobertura report generation code, determine an appropriate filename value, and verify that the generated report includes it and remains compatible with the selected DTD.

Written by the indexing model from the issue text.