librespeed.org has no AAAA record, so --ipv6 cannot fetch the server list

[BKPepe]

librespeed.org is IPv4-only, and it is where the CLI fetches
backend-servers/servers.php. On an IPv6-only host, or with --ipv6, the run
ends before it starts:

$ librespeed-cli --ipv6 --server 79
Error when fetching server list: Get "https://librespeed.org/backend-servers/servers.php":
dial tcp6: lookup librespeed.org: no such host

Most servers in that list are dual-stack, so the list itself is the only thing
blocking an IPv6 test:

librespeed.org                A=78.46.162.45   AAAA=—
de1.backend.librespeed.org    A=78.46.162.45   AAAA=—
speedtest.cesnet.cz           A=78.128.211.42  AAAA=2001:718:1:1f:50:56ff:feee:42
librespeed.turris.cz          A=217.31.192.72  AAAA=2001:1488:ac15:ff80::72

[sstidl]

DNS change, not code related