test: Remove use of "mitchellh/go-testing-interface" for stdlib. (#25640)

The stdlib testing package now includes this interface, so we can
remove our dependency on the external library.

testutil/server.go

@@ -22,13 +22,13 @@ import (
 	"net/http"
 	"os"
 	"os/exec"
+	"testing"
 	"time"
 
 	cleanhttp "github.com/hashicorp/go-cleanhttp"
 	"github.com/hashicorp/nomad/ci"
 	"github.com/hashicorp/nomad/helper/discover"
 	"github.com/hashicorp/nomad/helper/pointer"
-	testing "github.com/mitchellh/go-testing-interface"
 )
 
 // TestServerConfig is the main server configuration struct.
@@ -154,7 +154,7 @@ func defaultServerConfig() *TestServerConfig {
 type TestServer struct {
 	cmd    *exec.Cmd
 	Config *TestServerConfig
-	t      testing.T
+	t      testing.TB
 
 	HTTPAddr   string
 	SerfAddr   string
@@ -163,7 +163,7 @@ type TestServer struct {
 
 // NewTestServer creates a new TestServer, and makes a call to
 // an optional callback function to modify the configuration.
-func NewTestServer(t testing.T, cb ServerConfigCallback) *TestServer {
+func NewTestServer(t testing.TB, cb ServerConfigCallback) *TestServer {
 	path, err := discover.NomadExecutable()
 	if err != nil {
 		t.Skipf("nomad not found, skipping: %v", err)