integration: replace ad-hoc test timeouts with named constants

Categorised timeouts in integrationutil/timeouts.go remove the drift
opportunity between same-purpose budgets repeated across the suite.
The auth, cli, dns, derp, ssh, and tags tests are swept; acl, route,
and general tests follow in later commits alongside their other
ergonomic fixes.

integration/dns_test.go

@@ -67,7 +67,7 @@ func TestResolveMagicDNS(t *testing.T) {
 				for _, ip := range ips {
 					assert.Contains(ct, result, ip.String(), "IP %s should be found in DNS resolution result from %s to %s", ip.String(), client.Hostname(), peer.Hostname())
 				}
-			}, integrationutil.ScaledTimeout(30*time.Second), 2*time.Second)
+			}, integrationutil.StatusReadyTimeout, 2*time.Second)
 		}
 	}
 }