Add warning for slow hostname lookups on OS X

[eatkins]

I spent a lot of time debugging why it took 5 seconds to run tests each
time. It turns out that if the hostname is not set explicitly on os x,
then getaddrinfo takes 5 seconds to try (and fail) to resolve the dns
entry for the localhostname. This is easily fixed by setting the
hostname, but it is not at all easy to figure out that a slow hostname
lookup is the reason why tests are slow to start.

I don't know if this is a common issue on other platforms, so only issue
the warning on OS X.

Related to #1506

(See the guidelines for contributing, linked above)

[lightbend-cla-validator]

Hi @eatkins,

Thank you for your contribution! We really value the time you've taken to put this together.

Before we proceed with reviewing this pull request, please sign the Lightbend Contributors License Agreement:

http://www.lightbend.com/contribute/cla

[eatkins]

I signed the Lightbend Contributors Licence Agreement.

[dwijnand]

Given the only use of the hostname is to embed it into the JUnit results XML, what if we just avoid the whole pitfall and use "localhost"?

[eatkins]

@dwijnand My concern with changing the behavior is that users with multiple build agents may rely on the hostname field to determine on which machine their tests ran. An alternative approach would be to make the machine name configurable in the project settings and switch to using localhost by default. This would require existing users to update the machine name when they upgrade sbt though.

[dwijnand]

@eatkins OK! Do you want to resolve the merge conflict, or shall I?

[eatkins]

Resolved.

[dwijnand]

Thanks @eatkins!