Can't skip range request tests

[achingbrain]

The range request tests error if neither single or multiple ranges are supported.

Since there's no way to use range headers in the service worker gateway, they need to be skipped but they can't, since the error doesn't happen during the test.

Also the contentType value is set as a side-effect of running the "GET for /ipfs/ file with multiple range request returned HTTP 206" test which means the single/multiple range request tests can't be run in isolation.