ephemeral rustpkg test errors

[thestinger]

http://buildbot.rust-lang.org/builders/auto-mac-64-opt/builds/511/steps/test/logs/stdio

tests::rustpkg_library_target
tests::test_install_git
tests::test_package_version

I am going to xfail these for now, because they're causing a lot of buildbot failures.

[thestinger]

cc @catamorphism

[alexcrichton]

This might be related to #7925?

[thestinger]

@alexcrichton: possible, @catamorphism can close this as a duplicate if that's indeed the issue

[catamorphism]

I have a fix, it's just not checked in yet.

[catamorphism]

Fixed as of d9293d1