Hmm. It looks like I've misjudged how the test harness runs the tests somehow. I'll try again, as it makes little sense to me why it would run that test in http2 mode when I marked it to be skipped in http2 mode...
Bug 1397646 Comment 38 Edit History
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Hmm. It looks like I've missed another test or two which need to be split up so that they run differently in h2 mode (dom/tests/mochitest/fetch/test_fetch_basic_http.html and dom/tests/mochitest/fetch/test_fetch_basic_http_sw_empty_reroute.html). I'll try again when I have some more time.