Core: Refactor internal ProcessingQueue into class

This is motivated by the last remaining build warning from Rollup:

> src/qunit.js → qunit/qunit.js...
> (!) Circular dependency
> src/test.js -> src/core/processing-queue.js -> src/test.js
> created qunit/qunit.js in 2s

ProcessingQueue needs access to the `test` function defined in test.js.

Fix by turning the module into a class, that we create a singleton
of in core.js (with access to test.js) and then use that singleton
where we previously used ProcessingQueue statically.

Closes #1740.

src/core.js

@@ -31,6 +31,8 @@ const QUnit = {};
 // rather than partly in config.js and partly here.
 config.currentModule.suiteReport = runSuite;
 
+config.pq = new ProcessingQueue(test);
+
 let globalStartCalled = false;
 let runStarted = false;
 
@@ -157,7 +159,7 @@ function scheduleBegin () {
 
 function unblockAndAdvanceQueue () {
   config.blocking = false;
-  ProcessingQueue.advance();
+  config.pq.advance();
 }
 
 export function begin () {

src/core/config.js

@@ -99,6 +99,9 @@ const config = {
   // Ref https://github.com/qunitjs/qunit/pull/1598
   globalHooks: {},
 
+  // Internal: ProcessingQueue singleton, created in /src/core.js
+  pq: null,
+
   // Internal state
   blocking: true,
   callbacks: {},