ADBDEV-6534: Fix IPv6 checking in cdbsenddummypacket unit test

[silent-observer]

Fix IPv6 checking in cdbsenddummypacket unit test

cdbsenddummypacket unit test checks for IPv6 availability to skip IPv6-related
tests. However, the checking was not sufficient since if IPv6 is present in
the system but disabled. In that case socket() succeeds, causing the test to
believe IPv6 is available, but later bind() fails with errno = EADDRNOTAVAIL.

This patch fixes the test failing by implementing a more detailed check for
IPv6 availability. This is only necessary for manual runs of this test since
CI pipeline already has IPv6 enabled.

Note: commit b9cb522 is related, but the
complicated IPv6 check there is only necessary because it was used for regular
GPDB operation, requiring it to check non-loopback addresses too. This is not
necessary here because the tests only need loopback IPv6 address to pass, not
full IPv6 support.

---

To test manually, disable IPv6 on the host machine or run this inside a Docker container with IPv6 disabled. For example, on Ubuntu:

sudo sysctl -w net.ipv6.conf.all.disable_ipv6=1
sudo sysctl -w net.ipv6.conf.default.disable_ipv6=1
sudo sysctl -w net.ipv6.conf.lo.disable_ipv6=1