storage: Minor test improvements

[mdmower]

Resolve sinon warnings:

ERROR: '[Storage] Failed to load store:  intentional'
    The test "Storage   Storage should recover from binding failure" 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>]);'
ERROR: '[Storage] Failed to load store:  InvalidCharacterError: Failed to execute 'atob' on 'Window': The string to be decoded is not correctly encoded.'
    The test "Storage   Storage should recover from binding error" 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>]);'
ERROR: '[Storage] Error: localStorage not supported.'
    The test "LocalStorageBinding   should throw if localStorage is not supported" 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>]);'
ERROR: '[Storage] Error: localStorage not supported.'
    The test "LocalStorageBinding   should bypass loading from localStorage if getItem throws" 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>]);'
ERROR: '[Storage] Error: localStorage not supported.'
    The test "LocalStorageBinding   should bypass saving to localStorage if getItem throws" 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