Correctly ignore some tests on windows

These two tests are notoriously flaky on the windows bots right now, so I'm
ignoring them until I can investigate them some more. The truncate_works test
has been flaky for quite some time, but it has gotten much worse recently. The
test_exists test has been flaky since the recent std::run rewrite landed.
Finally, the "unix pipe" test failure is a recent discovery on the try bots. I
haven't seen this failing much, but better safe than sorry!

cc #12516

src/libstd/io/net/unix.rs

@@ -192,7 +192,7 @@ mod tests {
         }, proc(_client) {
             // drop the client
         })
-    })
+    } #[ignore(cfg(windows))]) // FIXME(#12516)
 
     iotest!(fn write_begone() {
         smalltest(proc(mut server) {