chrome.runtime.id should be a non-empty string

[nickserv]

Various libraries like webextension-polyfill expect chrome.runtime.id to be a non-empty string. This causes several issues integrating sinon-chrome such as mozilla/webextension-polyfill#236 and #103. While sinon-chrome can't guess the exact id, using something like "testid" (suggested in mozilla/webextension-polyfill#218 (comment)) seems to work pretty well. Alternatively we could use a random string of lowercase letters so it's not a hard coded value.