General fixups discovered by checking errors

There was a lot of tests that actually threw a lot of errors and that did
not pass all the way because we didnt check everything. This commit should
fix all of these cases.

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

integration/utils.go

@@ -1,6 +1,7 @@
 package integration
 
 import (
+	"strings"
 	"testing"
 	"time"
 
@@ -59,6 +60,13 @@ func assertNoErrLogout(t *testing.T, err error) {
 	assertNoErrf(t, "failed to log out tailscale nodes: %s", err)
 }
 
+func assertContains(t *testing.T, str, subStr string) {
+	t.Helper()
+	if !strings.Contains(str, subStr) {
+		t.Fatalf("%#v does not contain %#v", str, subStr)
+	}
+}
+
 func pingAllHelper(t *testing.T, clients []TailscaleClient, addrs []string) int {
 	t.Helper()
 	success := 0