Fix flaky registry test

[BrianJKoopman]

Description

Fix a flaky registry test.

Motivation and Context

This registry test would occasionally fail, as it was relying on catching the state of the main registry process in STOPPING to pass. The failure was that sometimes it'd still be RUNNING. The fix is to just wait for the process to end and check that it SUCCEEDED.

How Has This Been Tested?

Tested locally by running the test.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• Unless I am preparing a release, I have opened this PR onto the develop branch.