test: skip ipv6 tests when ipv6 is not supported

To verify on Linux, run `sudo sysctl net.ipv6.conf.all.disable_ipv6=1`
before the test suite.

Fixes: https://github.com/libuv/libuv/issues/10
Revert "unix: only set SO_REUSEADDR when bind() fails"

This reverts commit a9e95a33bc723dc42daea2fcde6dde1d4cc5efad.

This seems to cause regressions in the test suite when IPv6 is disabled.

As optimizations go, it's a very minor one so let's not spend too much
effort trying to track down the cause of the regressions but revert the
change before it makes its way into a release.