tstest/integration/vms: smoke test derphttp through mitm proxies

Updates #4377

Very smoky/high-level test to ensure that derphttp internals play well
with an agressive (stare + bump) meddler-in-the-middle proxy.

Signed-off-by: Tom DNetto <tom@tailscale.com>

tstest/integration/vms/vm_setup_test.go

@@ -23,6 +23,7 @@ import (
 	"strconv"
 	"strings"
 	"testing"
+	"time"
 
 	"github.com/aws/aws-sdk-go-v2/aws"
 	"github.com/aws/aws-sdk-go-v2/config"
@@ -49,6 +50,19 @@ func (vm *vmInstance) running() bool {
 	}
 }
 
+func (vm *vmInstance) waitStartup(t *testing.T) {
+	t.Helper()
+	for i := 0; i < 100; i++ {
+		if vm.running() {
+			break
+		}
+		time.Sleep(100 * time.Millisecond)
+	}
+	if !vm.running() {
+		t.Fatal("vm not running")
+	}
+}
+
 func (h *Harness) makeImage(t *testing.T, d Distro, cdir string) string {
 	if !strings.HasPrefix(d.Name, "nixos") {
 		t.Fatal("image generation for non-nixos is not implemented")