net: pass tests on Plan 9

Ignore problematic tests and un-ignore one
we can now pass.

R=rsc, rminnich, ality, dave, bradfitz
CC=golang-dev
https://golang.org/cl/7396060

src/pkg/net/file_test.go

@@ -89,7 +89,7 @@ var fileListenerTests = []struct {
 
 func TestFileListener(t *testing.T) {
 	switch runtime.GOOS {
-	case "plan9", "windows":
+	case "windows":
 		t.Skipf("skipping test on %q", runtime.GOOS)
 	}