test: findUserArg has no unit tests

[ishwar170695]

findUserArg in singleContainer.ts is exported but has no test coverage.
No test file currently imports from singleContainer.

Proposing to add src/test/singleContainer.test.ts covering these cases:

• ['-u', 'myuser'] -> 'myuser'
• ['--user', 'myuser'] -> 'myuser'
• ['--user=myuser'] -> 'myuser'
• ['--user', 'a', '-u', 'b'] -> 'b' (last wins)
• [] -> undefined

Would this be helpful?

[ishwar170695]

Is there a reason test coverage for findUserArg hasn't been prioritized like design decision, or just hasn't come up yet?