test: Add pytest infrastructure and attachment tests

In making my last fix to attachments, I found it challenging not
having tests to ensure there was no regression.

Added pytest with minimal setup and isolated configuration. Created
a separate test workflow to keep tests isolated from linting.

Tests cover the key elements of the attachment logic:
- URL extraction from issue bodies
- Filename extraction from different URL types
- Filename collision resolution
- Manifest duplicate prevention

tests/__init__.py

@@ -0,0 +1 @@
+"""Tests for python-github-backup."""