Our test servers run either on ipv4 *or* on ipv6, as requested.
A test case using 'localhost' or '*.local' in the url needs to
run with the specific version of the server started.
Otherwise, curl's "happy eyeball"ing will connect to another
server that may be running due to parallel testing or for some
other reasons.
Note that port reuse here depends on the OS strategy and it
seems netbsd is especially likely to hit this.
Closes#15060
