add "line numbers do/do not render" tests, update snapshots after bugfix

Author: simmerer

__tests__/__snapshots__/light-async.js.snap

@@ -53,21 +53,6 @@ exports[`SyntaxHighlighter render as text if language doesnt exist 1`] = `
       }
     }
   >
-    <span
-      className="linenumber react-syntax-highlighter-line-number"
-      style={
-        Object {
-          "color": "slategray",
-          "display": "inline-block",
-          "minWidth": "1em",
-          "paddingRight": "1em",
-          "textAlign": "right",
-          "userSelect": "none",
-        }
-      }
-    >
-      1
-    </span>
     <span
       style={Object {}}
     >