JSON RPC client testing improvements

[ellbosch]

Happy Thanksgiving!

This PR improves our testing with JSON RPC client. Changes specifically address feedback from the Weekly Tech Review.

Changes:

• Addresses a hopeful fix for the flaky test that sometimes failed to shut down a thread.
• Raises assertion errors when exceptions should've been caught but didn't.

In thanks to these changes, I discovered that one of our tests fails to catch an exception when it should've. I raised an issue at #371 and have marked that specific test method to skip on runtime.

Tag: #281

[udeeshagautam]

You might want to re-run PR build