It was a painful amount of work to generate the actual payload.
Since the only difference was a small build URL, I manually
edited the payload and used that for testing.
This commit gets our GitHub webhook up to 100% test coverage.
This is a part of our efforts to get this webhook's coverage
up to 100%.
Note that apart from just testing an uncovered line of code, this
commit also fixes a minor bug in the code for messages about issue
comment deletion and editing.
All webhook fixtures in zerver/fixtures/<webhook_name> have now
been moved to dedicated webhook-specific directories under
zerver/webhooks/<webhook_name>/fixtures, where <webhook_name> is
the name of the webhook.