chore(bigquery): re-enable load test (#5025)

Fixes: https://github.com/googleapis/google-cloud-go/issues/4418
diff --git a/bigquery/integration_test.go b/bigquery/integration_test.go
index bbcf261..b484382 100644
--- a/bigquery/integration_test.go
+++ b/bigquery/integration_test.go
@@ -1951,7 +1951,6 @@
 }
 
 func TestIntegration_Load(t *testing.T) {
-	t.Skip("https://github.com/googleapis/google-cloud-go/issues/4418")
 	if client == nil {
 		t.Skip("Integration tests skipped")
 	}