Worker: fix sync issue in TestUpstreamBufferManagerUnavailable unit test

Fix synchronisation/goroutine issue in the "upstream buffer" test,
where very occasionally the queue size was checked at the wrong time.

internal/worker/upstream_buffer_test.go

@@ -6,6 +6,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"sync"
 	"testing"
 
 	"github.com/benbjohnson/clock"
@@ -83,17 +84,20 @@ func TestUpstreamBufferManagerUnavailable(t *testing.T) {
 	assert.Equal(t, 1, queueSize)
 
 	// Wait for the flushing with Manager available.
-	resp := &api.TaskUpdateResponse{}
+	wg := sync.WaitGroup{}
+	wg.Add(1)
 	mocks.client.EXPECT().
 		TaskUpdateWithResponse(ctx, taskID, update).
-		Return(resp, nil).
+		DoAndReturn(func(ctx context.Context, taskID string, body api.TaskUpdateJSONRequestBody, editors ...api.RequestEditorFn) (*api.TaskUpdateResponse, error) {
+			wg.Done()
+			return &api.TaskUpdateResponse{}, nil
+		}).
 		After(managerCallFail)
 
-	// Only add exactly the flush interval, as that maximises the chances of
-	// getting conflicts on the database level (if we didn't have the
-	// database-protection mutex).
 	mocks.clock.Add(defaultUpstreamFlushInterval)
 
+	wg.Wait()
+
 	// Queue should be empty now.
 	ub.dbMutex.Lock()
 	queueSize, err = ub.queueSize()