tstest/integration/vms: split up test framework into files (#2396)

My spatial memory functions poorly with large files and the vms_test.go
file recently surpassed the point where it functions adequately. This
patch splits up vms_test.go into more files to make my spatial memory
function like I need it to.

Signed-off-by: Christine Dodrill <xe@tailscale.com>

tstest/integration/vms/derive_bindhost_test.go

@@ -0,0 +1,50 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build linux
+
+package vms
+
+import (
+	"io"
+	"testing"
+
+	"inet.af/netaddr"
+	"tailscale.com/net/interfaces"
+)
+
+func deriveBindhost(t *testing.T) string {
+	t.Helper()
+
+	ifName, err := interfaces.DefaultRouteInterface()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var ret string
+	err = interfaces.ForeachInterfaceAddress(func(i interfaces.Interface, prefix netaddr.IPPrefix) {
+		if ret != "" || i.Name != ifName {
+			return
+		}
+		ret = prefix.IP().String()
+	})
+	if ret != "" {
+		return ret
+	}
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Fatal("can't find a bindhost")
+	return "unreachable"
+}
+
+func TestDeriveBindhost(t *testing.T) {
+	t.Log(deriveBindhost(t))
+}
+
+type nopWriteCloser struct {
+	io.Writer
+}
+
+func (nwc nopWriteCloser) Close() error { return nil }