Enable uv pipe tests on windows

Turns out the pipe names must have special names on windows. Once we have
special names, all the tests pass just fine.

Closes #10386
auto merge of #10409 : alexcrichton/rust/issue-10386, r=brson

Turns out the pipe names must have special names on windows. Once we have
special names, all the tests pass just fine.

Closes #10386