fixed race condition in tests assuming TEST_EVENT_OBSERVER_SKIP_RETRY…

[rdeioris]

Description

This patch fixes a race condition in the event_dispatcher tests, specifically:

event_dispatcher::test::test_process_pending_payloads
event_dispatcher::test::test_send_payload_timeout
event_dispatcher::test::test_send_payload_with_db

When executed in parallel, the TEST_EVENT_OBSERVER_SKIP_RETRY global mutex is used for enabling/disabling the retry system for events (by storing them in the db for eventually retry their sending to the observer).

This global is only used in the tests and assumed to be off. The problem is for those tests that are executed while TEST_EVENT_OBSERVER_SKIP_RETRY is set to true by another test and do not lock it before reading.

The patch simply enforces locking (where missing) before sending the payload in those tests.

Applicable issues

Additional info (benefits, drawbacks, caveats)

This originally was part (in a more complex form) of #5570 .

Checklist

• Test coverage for new or modified code paths
• Changelog is updated
• Required documentation changes (e.g., docs/rpc/openapi.yaml and rpc-endpoints.md for v2 endpoints, event-dispatcher.md for new events)
• New clarity functions have corresponding PR in clarity-benchmarking repo
• New integration test(s) added to bitcoin-tests.yml