Always close test HTTP server during cleanup (#2261)

## Changes
This PR registers the `server.Close()` function to be run during test
cleanup in the server initialization function. This ensures that all
test servers are closed as soon as the test they are scoped to finish.

Motivated by https://github.com/databricks/cli/pull/2255/files where a
regression was introduced where we did not close the test server.

## Tests
N/A

acceptance/cmd_server_test.go

@@ -13,7 +13,8 @@ import (
 )
 
 func StartCmdServer(t *testing.T) *testserver.Server {
-	server := StartServer(t)
+	server := testserver.New(t)
+
 	server.Handle("/", func(r *http.Request) (any, error) {
 		q := r.URL.Query()
 		args := strings.Split(q.Get("args"), " ")

acceptance/server_test.go

@@ -2,7 +2,6 @@ package acceptance_test
 
 import (
 	"net/http"
-	"testing"
 
 	"github.com/databricks/cli/libs/testserver"
 	"github.com/databricks/databricks-sdk-go/service/catalog"
@@ -11,14 +10,6 @@ import (
 	"github.com/databricks/databricks-sdk-go/service/workspace"
 )
 
-func StartServer(t *testing.T) *testserver.Server {
-	server := testserver.New(t)
-	t.Cleanup(func() {
-		server.Close()
-	})
-	return server
-}
-
 func AddHandlers(server *testserver.Server) {
 	server.Handle("GET /api/2.0/policies/clusters/list", func(r *http.Request) (any, error) {
 		return compute.ListPoliciesResponse{

libs/testserver/server.go

@@ -18,6 +18,7 @@ type Server struct {
 func New(t testutil.TestingT) *Server {
 	mux := http.NewServeMux()
 	server := httptest.NewServer(mux)
+	t.Cleanup(server.Close)
 
 	return &Server{
 		Server: server,
@@ -28,10 +29,6 @@ func New(t testutil.TestingT) *Server {
 
 type HandlerFunc func(req *http.Request) (resp any, err error)
 
-func (s *Server) Close() {
-	s.Server.Close()
-}
-
 func (s *Server) Handle(pattern string, handler HandlerFunc) {
 	s.Mux.HandleFunc(pattern, func(w http.ResponseWriter, r *http.Request) {
 		resp, err := handler(r)