Bug 934640 - Let 'Ensure that databases() returns a promise' test case accept a rejected Promise to match its description. r=dom-workers-and-storage-reviewers,ytausky

Before the patch, the test failed if indexedDB.databases() returns a promise, but the promise was rejected. This was not what "Ensure that databases() returns a promise." describes. Therefore, either the description should be changed, or the test. But there are other tests that check if the promise is resolved, I changed the test.

Differential Revision: https://phabricator.services.mozilla.com/D54142

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extra : moz-landing-system : lando