tests: make testagent shutdown idempotent

Avoid double freeing ports if an agent.Shutdown() is called multiple
times.

command/agent/testagent.go

@@ -84,6 +84,9 @@ type TestAgent struct {
 
 	// Enterprise specifies if the agent is enterprise or not
 	Enterprise bool
+
+	// shutdown is set to true if agent has been shutdown
+	shutdown bool
 }
 
 // NewTestAgent returns a started agent with the given name and
@@ -259,6 +262,11 @@ func (a *TestAgent) start() (*Agent, error) {
 // Shutdown stops the agent and removes the data directory if it is
 // managed by the test agent.
 func (a *TestAgent) Shutdown() error {
+	if a.shutdown {
+		return nil
+	}
+	a.shutdown = true
+
 	defer freeport.Return(a.ports)
 
 	defer func() {