sdk: Use /v1/status/leader endpoint when starting a test server (#8192)

Switch from /v1/agent/self to /v1/status/leader when checking if the test server has come up successfully in the waitForAPI function.

Previously, the test server was relying (probably not intentionally) on the default value of the acl_enforce_version_8 in the TestConfig, which was false. So if you create a test server and enabled ACLs, they would not be enforced and the server would be able to come up pretty quickly because /v1/agent/self would return a 200 status pretty much as soon as the agent is running and most likely before leader election is finished.

Now that we have removed acl_enforce_version_8 property (equivalent to being true by default) if you've created a test server with ACLs enabled, it will need to wait for leader election and for ACLs to be initialized before it'll get a successful response from the /v1/agent/self.

Note: With this change, waitForAPI function no longer requires a 200 response status from the v1/status/leader endpoint. This is because in some tests, namely TestAPI_AgentLeave, we are only running clients, and this endpoint returns a 500 status.

sdk/testutil/server.go

@@ -366,9 +366,12 @@ func (s *TestServer) Stop() error {
 	return s.cmd.Wait()
 }
 
-// waitForAPI waits for only the agent HTTP endpoint to start
+// waitForAPI waits for the /status/leader HTTP endpoint to start
 // responding. This is an indication that the agent has started,
 // but will likely return before a leader is elected.
+// Note: We do not check for a successful response status because
+// we want this function to return without error even when
+// there's no leader elected.
 func (s *TestServer) waitForAPI() error {
 	var failed bool
 
@@ -380,18 +383,13 @@ func (s *TestServer) waitForAPI() error {
 	for !time.Now().After(deadline) {
 		time.Sleep(timer.Wait)
 
-		url := s.url("/v1/agent/self")
-		resp, err := s.masterGet(url)
+		url := s.url("/v1/status/leader")
+		_, err := s.masterGet(url)
 		if err != nil {
 			failed = true
 			continue
 		}
-		resp.Body.Close()
 
-		if err = s.requireOK(resp); err != nil {
-			failed = true
-			continue
-		}
 		failed = false
 	}
 	if failed {