Summarize non-content-checked replies in report

[mthuurne]

It would be useful to show, at the top of the report, how many replies did not have their content checked. Preferably split by reason.

Possible reasons for skipping a content check include:

• content checks were disabled for the received HTTP status code
• no content checker was available or enabled for the returned content type

[mthuurne]

Pages for which none of the queries were content-checked could be categorized as "skipped" in the JUnit XML report.

The field Report.checked could be used to track whether a query was content-checked, but it is currently set to True if content checking is disabled for the returned HTTP status code, which contradicts its name. I think that what Report.checked is actually used for at the moment is avoiding a redundant message in the report if a specific reason was logged earlier.