Issue #6207: Add xpath regression test for AnonInnerLength

[mincong-h]

This PR adds XPath regression test for suppression filter on AnonInnerLength check, see #6207.

[rnveach]

All input files should start with Input.

[romani]

I am not remember why ... but we do not do this notations in this part of code.
Examples:
https://github.com/checkstyle/checkstyle/tree/master/src/it/resources/org/checkstyle/suppressionxpathfilter/classmemberimpliedmodifier
https://github.com/checkstyle/checkstyle/tree/master/src/it/resources/org/checkstyle/suppressionxpathfilter/emptyforiteratorpad

Do you remember why ? We definitely did this on purpose.

[romani]

non of existing is named like "InputXxxxxxxx" ....
@timurt , do you remember why we did not followed general rule for input test java files ?

@rnveach , I think it should not be blocker for this PRs, as find a reason, or no reason, we can improve in next PRs.

[romani]

PR is merged, but if @timurt share reason, it would be good.

[timurt]

@romani it seems that idea was to separate tests and inputs for Checks and for Xpath Generator I guess

I do not actually remember whole context, but please read this discussion, I think it all came from there :)