Add unit tests for client/activation/serviceRegistry.ts

[kimadeline]

For #6163

Coverage went down(?) on client/common/logger.ts, issue to add unit tests here: #6099

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Has a news entry file (remember to thank yourself!)
• [n/a] Appropriate comments and documentation strings in the code
• [n/a] Has sufficient logging.
• [n/a] Has telemetry for enhancements.
• Unit tests & system/integration tests are added/updated
• [n/a] Test plan is updated as appropriate
• [n/a] package-lock.json has been regenerated by running npm install (if dependencies have changed)
• [n/a] The wiki is updated with any design decisions/details.

[karrtikr]

Wonder why the coverage went down

[kimadeline]

Yeah I don't understand either 🤷‍♀

The file I worked on now has 100% coverage:

But Coveralls says that the coverage in client/common/logger.ts went down, even though I didn't touch this file (Coveralls already knew it existed, we have an open issue to to add unit tests for it: #6099) 🤔

Oh well, thank you for the review 👍