- ignore Cask's files in `readall` (for now, there's an intentional
  syntax error that will need fixed)
- run Cask's tests if they exist
- don't check Cask's files in coverage reports (for now)

This basically started once our integration tests caused the overall
test time to raise above 10 minutes, causing some coverage data to be
dropped because SimpleCov believed it to be stale.

Make `Library/Homebrew` the new root now that *all* implementation files
are in this directory. Update filters/groups accordingly.

Note that paths in filters/groups are not anchored at the root and are
always matched against the full path. Our rules are effectively anchored
because they always include the `/Homebrew/` path component.

There is no good reason to configure the options specific to integration
tests in a completely different location from all other options.

This will almost certainly fail every time so let's disable this error.

Groups make it easier to get an overview of the coverage without having
to scan through a single very long list of files. They also display a
possibly helpful per-group coverage.

The project name is normally derived from the `root` directory which
happens to be `Library` in our case, thus make it explicitly `Homebrew`.

Closes Homebrew/homebrew#49303.

Signed-off-by: Martin Afanasjew <martin@afanasjew.de>