service: Minor test improvement

[mdmower]

Resolve sinon warning:

ERROR: '[SERVICE] failed to dispose service b intentional'
    The test "service ampdoc singletons should dispose disposable services" resulted in a call to console.error. (See above line.)
    ⤷ If the error is not expected, fix the code that generated the error.
    ⤷ If the error is expected (and synchronous), use the following pattern to wrap the test code that generated the error:
        'allowConsoleError(() => { <code that generated the error> });'
    ⤷ If the error is expected (and asynchronous), use the following pattern at the top of the test:
        'expectAsyncConsoleError(<string or regex>[, <number of times the error message repeats>]);'

Full log: https://gist.github.com/mdmower/d2252c6dc06d096687c56a6368a30c41