test: move WPT to its own testing module

This is first in a hoped-for series of moves away from a monolithic
common.js that is loaded for every test and towards a more modular
approach. (In the end, common.js will hopefully contain checks for
variables leaking into the global space and perhaps some of the more
ubiquitous functions like common.mustCall().)

Move the WPT testing code to its own module.

PR-URL: https://github.com/nodejs/node/pull/12736
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>

test/sequential/test-module-loading.js

@@ -243,7 +243,7 @@ try {
   }, {});
 
   assert.deepStrictEqual(children, {
-    'common.js': {},
+    'common/index.js': {},
     'fixtures/not-main-module.js': {},
     'fixtures/a.js': {
       'fixtures/b/c.js': {