tests: make test-serve works on system that allow user to bind low port
It is possible to encounter linux system configured in a way that allow port
< 1024 to be bound by any users. So we update a test to focus more on the actual
test: "resolving `daytime` to an actual port number.
The new auto-scaling CI runner provided by clever cloud is doing so. This is the
initial motivation for this change, but it seems useful anyway.
Differential Revision: https://phab.mercurial-scm.org/D11228
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