fix latest CI breakages by forking code from grpc-go's interop/testing

Also moves testing package to internal/testing

internal/testing/cmd/testserver/unix.go

@@ -0,0 +1,17 @@
+// +build darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package main
+
+import "flag"
+
+var (
+	unix = flag.String("unix", "",
+		`Use instead of -p to indicate listening on a Unix domain socket instead of a
+    	TCP port. If present, must be the path to a domain socket.`)
+)
+
+func init() {
+	getUnixSocket = func() string {
+		return *unix
+	}
+}