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/integration_test.go

@@ -4,6 +4,8 @@
 
 package integration
 
+//go:generate go run gen_deps.go
+
 import (
 	"bytes"
 	"context"