Fix last remaining test on Android

[finagolfin]

Motivation:

Get all tests passing on Android AArch64

Modifications:

• Check if localhost is set for IPv6 in BootstrapTest/testClientBindWorksOnSocketsBoundToEitherIPv4OrIPv6Only, and use ip6-localhost if not

Result:

All tests pass natively on Android AArch64

The problem is that the localhost definitions in /etc/hosts on Android look like this, so I've long disabled this test on my Android CI:

> ag localhost /etc/hosts
127.0.0.1       localhost
::1             ip6-localhost

Whereas this test passed on linux because it defines localhost for both protocols:

> ag localhost /etc/hosts
127.0.0.1 localhost
::1 localhost ip6-localhost ip6-loopback

Binging it online, I see that Darwin doesn't appear to define ip6-localhost, so I kept that the same. I only tested this change natively on Android, so let's see if the CI catches anything on linux and macOS.

[Lukasa]

Thanks for this patch!

I worry about this code being a little brittle. Can I suggest that we instead do a DNS resolution on localhost and ip6-localhost to check whether the names are defined, and what they resolve to? That will make this code a lot more predictable, and work on systems regardless of how specifically they've handled /etc/hosts.

[finagolfin]

Made the DNS resolution change you asked for, let me know what you think.