Add back a flatten file list on the coverage page

[sjoubert]

Bug report

Expected Behavior

Before the coverage page revamp from #3348 the files were listed with their full path. This allowed by combining the "All" button and the coverage percent column sort to easily find the lowest covered files in large repo.
With the current tree view one has to manually and recursively navigate the whole hierarchy and remember which files have low coverage.

I like the tree view by default, but a flatten view to combine with sort operations would be useful.

Actual Behavior

Only the tree view/navigation is available.

CDash Version

v4.8.0

Additional Information

I've filed this as "bug report" since it's a regression in a user story, but feel free to label it as "feature request" if needed.