Add hidden file to Filter/AcceptTest

[jnoordsij]

Description

When debugging behavior on treatment of hidden files, I discovered after some testing the following behavior:

• on 3.x hidden files are always ignored, even if explicitly added by full path
• on 4.x hidden files are ignored on recursive traversal, but processed when explicitly added by full path

As I could not find any change that intended to change this behavior,
I was wondering if this was entirely intended,
but it seems to match with the suggested approach in #395.
For what I have gathered, I think this behavioral change was a potentially unintended side-effect of adding processing of files without extensions in 4.x.
Given that I think this new behavior has been around since and better matches user expectation,
I've opted to create two tests, one here (3.x) and one for 4.x (see #1425) to actively verify this behavior.

Note: if you no longer intend to accept tests on 3.x, I will gladly rebase to 4.x.

Suggested changelog entry

n/a

Related issues/external references

Related to #395

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
  • This change is only breaking for integrators, not for external standards or end-users.
• Documentation improvement
• None of the above; just a new behavior test

PR checklist

• I have checked there is no other PR open for the same change.
• I have read the Contribution Guidelines.
• I grant the project the right to include and distribute the code under the BSD-3-Clause license (and I have the right to grant these rights).
• I have added tests to cover my changes.
• I have verified that the code complies with the projects coding standards.
• [Required for new sniffs] I have added XML documentation for the sniff.
• I have opened a sister-PR in the documentation repository to update the Wiki.