The `thread_tests/yields_execution` test case in FC sometimes fails

**Bug Description**

The `thread_tests/yields_execution` test case in FC sometimes fails, E.G. https://github.com/bitshares/bitshares-fc/runs/3862984117?check_suite_focus=true .

**Impacts**
Describe which portion(s) of BitShares Core may be impacted by this bug. Please tick at least one box.
- [ ] API (the application programming interface)
- [ ] Build (the build process or something prior to compiled code)
- [ ] CLI (the command line wallet)
- [ ] Deployment (the deployment process after building such as Docker, Travis, etc.)
- [ ] DEX (the Decentralized EXchange, market engine, etc.)
- [ ] P2P (the peer-to-peer network for transaction/block propagation)
- [ ] Performance (system or user efficiency, etc.)
- [ ] Protocol (the blockchain logic, consensus, validation, etc.)
- [ ] Security (the security of system or user data, etc.)
- [ ] UX (the User Experience)
- [ ] Other (please add below)

**Host Environment**
Please provide details about the host environment. Much of this information can be found running: `witness_node --version`.
- Host OS: [e.g. Ubuntu 18.04 LTS]
- Host Physical RAM [e.g. 4GB]
- BitShares Version: [e.g. 2.0.180425]
- OpenSSL Version: [e.g. 1.1.0g]
- Boost Version: [e.g. 1.65.1]

## CORE TEAM TASK LIST
- [ ] Evaluate / Prioritize Bug Report
- [ ] Refine User Stories / Requirements
- [ ] Define Test Cases
- [ ] Design / Develop Solution
- [ ] Perform QA/Testing
- [ ] Update Documentation

Start with the FC `thread_tests/yields_execution` test case and reproduce it in the linked GitHub Actions run or the local test suite. Trace why the test fails intermittently and make the test reliable, with the full relevant test suite passing consistently.

Written by the indexing model from the issue text.