Adding tests for Matter custom xml support

[dhchandw]

Added tests for Matter custom xml support for the following use cases:

• loads a file with a custom cluster and an extension of a cluster and checks it
• loads a second file with a further extension of the extended cluster and checks it
• tests generation with custom xml
• loading bad custom xml -> throw warning
• Add 2 custom files with manufacturer name/code and attribute code collisions -> throw error.
  • This should be an SQL error, it is currently broken and should be fixed in PR Adding constraints to the database tables to avoid redundant entries #1352 is merged.
• test generation after deleting the custom xml
  • This doesn't work as expected - Custom Cluster Remains in Generated Files After Custom XML Removal #1387
• Add custom xml, make changes, delete, add again -> make sure that generated content has new edits
  • Effected by Custom Cluster Remains in Generated Files After Custom XML Removal #1387 and Custom XML bug in the following use case - Add custom xml, make changes, delete, add again (both in ZigBee and Matter) #1391
• Loading zap file with custom xml that does not exist - throw error
• Loading zap file with custom xml and generating

Once PR #1352 is merged and #1387 and #1391 are resolved - tests should be updated appropriately.

[brdandu]

• One edge case worth consideration is if the custom xml path is valid but does not exist in the database. what happens? We have users sometime deleting the .zap repo and then opening their .zap files. Ideally that behavior should not break ZAP. Worth testing that as well. Approving the PR however if you want to merge and add that later.
• Also link this PR to your issue for easy reference in the future.
• You have pretty good description on the PR. Add that to your git commit message it is useful information. Also add the git/jira issue associated with your PR to the git commit eg(JIRA: ZAPP-123 or Github: ZAP#123)