If there is at least one record for expected I/O, but all of them have
`ignoreIfUnused: true`, and no I/O is reported, then the tests should
succeed (they are all unused and thus ignored). There's a check which
expects to see I/O markers in that case, so this patch conditionally
skips it. (Also: fix some pre-existing typos pointed out in review.)
Differential Revision: https://phabricator.services.mozilla.com/D212632
