tstest/integration: build binaries only once

The existing code relied on the Go build cache to avoid
needless work when obtaining the tailscale binaries.

For non-obvious reasons, the binaries were getting re-linked
every time, which added 600ms or so on my machine to every test.

Instead, build the binaries exactly once, on demand.
This reduces the time to run 'go test -count=5' from 34s to 10s
on my machine.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

tstest/integration/vms/vms_test.go

@@ -30,6 +30,7 @@ import (
 	"golang.org/x/sync/semaphore"
 	"inet.af/netaddr"
 	"tailscale.com/tstest"
+	"tailscale.com/tstest/integration"
 	"tailscale.com/types/logger"
 )
 
@@ -52,6 +53,13 @@ var (
 	}()
 )
 
+func TestMain(m *testing.M) {
+	flag.Parse()
+	v := m.Run()
+	integration.CleanupBinaries()
+	os.Exit(v)
+}
+
 func TestDownloadImages(t *testing.T) {
 	if !*runVMTests {
 		t.Skip("not running integration tests (need --run-vm-tests)")