test: fix TestAgent.Start() to not segfault if the DNSServer cannot L…

…istenAndServe (hashicorp#6409)

The embedded `Server` field on a `DNSServer` is only set inside of the
`ListenAndServe` method. If that method fails for reasons like the
address being in use and is not bindable, then the `Server` field will
not be set and the overall `Agent.Start()` will fail.

This will trigger the inner loop of `TestAgent.Start()` to invoke
`ShutdownEndpoints` which will attempt to pretty print the DNS servers
using fields on that inner `Server` field. Because it was never set,
this causes a nil pointer dereference and crashes the test.

agent/agent.go

@@ -1654,8 +1654,10 @@ func (a *Agent) ShutdownEndpoints() {
 	}
 
 	for _, srv := range a.dnsServers {
-		a.logger.Printf("[INFO] agent: Stopping DNS server %s (%s)", srv.Server.Addr, srv.Server.Net)
-		srv.Shutdown()
+		if srv.Server != nil {
+			a.logger.Printf("[INFO] agent: Stopping DNS server %s (%s)", srv.Server.Addr, srv.Server.Net)
+			srv.Shutdown()
+		}
 	}
 	a.dnsServers = nil
 

agent/testagent.go

@@ -165,7 +165,7 @@ func (a *TestAgent) Start(t *testing.T) *TestAgent {
 			a.Agent = agent
 			break
 		} else if i == 0 {
-			require.Fail("%s %s Error starting agent: %s", id, a.Name, err)
+			require.Failf("%s %s Error starting agent: %s", id, a.Name, err)
 		} else if a.ExpectConfigError {
 			// Panic the error since this can be caught if needed. Pretty gross way to
 			// detect errors but enough for now and this is a tiny edge case that I'd