test: make TestAgent setup reusable

Pick the random ports only once and try starting with them
a number of times so that the configuration can be re-used.
This is because the ports are written into the data files
and a subsequent agent reading the files needs to have the
same ports.

For the same reason we do not remove the data directory on
every attempt since this makes it impossible to re-read the
data files.

command/agent/testagent.go

@@ -121,16 +121,12 @@ func (a *TestAgent) Start() *TestAgent {
 		a.Config.DataDir = d
 	}
 	id := UniqueID()
+
+	// since the ports are written into the data files we pick random ports
+	// only once and try a number of times to start with them.
+	pickRandomPorts(a.Config)
+
 	for i := 10; i >= 0; i-- {
-		pickRandomPorts(a.Config)
-
-		// ports are baked into the data files so we need to clear out the
-		// data dir on every retry
-		os.RemoveAll(a.Config.DataDir)
-		if err := os.MkdirAll(a.Config.DataDir, 0755); err != nil {
-			panic(fmt.Sprintf("Error creating dir %s: %s", a.Config.DataDir, err))
-		}
-
 		// write the keyring
 		if a.Key != "" {
 			writeKey := func(key, filename string) {