tstest/integration: help bust cmd/go's test caching

It was caching too aggressively, as it didn't see our deps due to our
running "go install tailscaled" as a child process.

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/integration/gen_deps.go

@@ -0,0 +1,53 @@
+// 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 ignore
+
+package main
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"log"
+	"os/exec"
+)
+
+func main() {
+	var x struct {
+		Imports []string
+	}
+	j, err := exec.Command("go", "list", "-json", "tailscale.com/cmd/tailscaled").Output()
+	if err != nil {
+		log.Fatal(err)
+	}
+	if err := json.Unmarshal(j, &x); err != nil {
+		log.Fatal(err)
+	}
+	var out bytes.Buffer
+	out.WriteString(`// 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.
+
+// Code generated by gen_deps.go; DO NOT EDIT.
+
+package integration
+
+import (
+	// And depend on a bunch of tailscaled innards, for Go's test caching.
+	// Otherwise cmd/go never sees that we depend on these packages'
+	// transitive deps when we run "go install tailscaled" in a child
+	// process and can cache a prior success when a dependency changes.
+`)
+	for _, dep := range x.Imports {
+		fmt.Fprintf(&out, "\t_ %q\n", dep)
+	}
+	fmt.Fprintf(&out, ")\n")
+
+	err = ioutil.WriteFile("tailscaled_deps_test.go", out.Bytes(), 0644)
+	if err != nil {
+		log.Fatal(err)
+	}
+}