add mutex for test field

Signed-off-by: Meredith Lancaster <malancas@github.com>

pkg/cmd/attestation/api/mock_httpClient_test.go

@@ -5,18 +5,22 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"sync"
 
 	"github.com/cli/cli/v2/pkg/cmd/attestation/test/data"
 	"github.com/golang/snappy"
 )
 
 type mockHttpClient struct {
+	mutex  sync.RWMutex
 	called bool
 	OnGet  func(url string) (*http.Response, error)
 }
 
 func (m *mockHttpClient) Get(url string) (*http.Response, error) {
+	m.mutex.Lock()
 	m.called = true
+	m.mutex.Unlock()
 	return m.OnGet(url)
 }