Remove unused constants from ol/webgl

And change the `EXTENSIONS` constant to a function

test/.eslintrc

@@ -5,7 +5,6 @@
   "globals": {
     "IMAGE_TOLERANCE": false,
     "afterLoadText": false,
-    "assertWebGL": false,
     "createMapDiv": true,
     "disposeMap": true,
     "expect": false,

test/test-extensions.js

@@ -1,5 +1,4 @@
 import {equals} from '../src/ol/array.js';
-import {WEBGL} from '../src/ol/has.js';
 // avoid importing anything that results in an instanceof check
 // since these extensions are global, instanceof checks fail with modules
 
@@ -376,12 +375,6 @@ import {WEBGL} from '../src/ol/has.js';
     map.dispose();
   };
 
-  global.assertWebGL = function(map) {
-    if (!WEBGL) {
-      expect().fail('No WebGL support!');
-    }
-  };
-
   function resembleCanvas(canvas, referenceImage, tolerance, done) {
     if (showMap) {
       const wrapper = document.createElement('div');