The test's whole value is that its fixture keeps the navigation settle loop
pumping, but nothing checked the fixture was reached. Pointing `page_url` at a
dead port passed in 0.6s instead of 1.25s, and that degraded version no longer
reproduced the #430 abort on the pre-fix tree -- so a fixture regression would
have turned this back into the `data:`-URL test it was written to replace,
silently and while staying green.
Count the requests the fixture answers and assert on it. Two per client, not
three: the engine does not fetch the `<img>`, only the document and /slow.js.
Verified the assertion kills the dead-port mutant (0 requests, fails) and that
the real run is stable across repeats.
Refs #430.
sycatle
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