[test] class names can't start with a digit

Try `document.querySelector('.109ja448cfod')` it will throw.
Discovered in https://github.com/mui-org/material-ui-x/blob/91b05ee8b6cc9fb1beab96187fd4308d4b7b294b/packages/grid/x-grid/src/XGrid.test.tsx#L71

test/utils/describeConformance.js

@@ -49,7 +49,7 @@ function findRootComponent(wrapper, { component }) {
 }
 
 function randomStringValue() {
-  return Math.random().toString(36).slice(2);
+  return `r${Math.random().toString(36).slice(2)}`;
 }
 
 /**