update runtime thread tests

Threads are now removed after every "inner" step. Any thread that
reaches its DONE conditions is immediately removed instead of requiring
an extra step to remove it. As such tests that check the number of
threads have been updated to consider this.

src/engine/runtime.js

@@ -188,6 +188,13 @@ class Runtime extends EventEmitter {
          */
         this._nonMonitorThreadCount = 0;
 
+        /**
+         * All threads that finished running and were removed from this.threads
+         * by behaviour in Sequencer.stepThreads.
+         * @type {Array<Thread>}
+         */
+        this._lastStepDoneThreads = null;
+
         /**
          * Currently known number of clones, used to enforce clone limit.
          * @type {number}
@@ -1560,6 +1567,9 @@ class Runtime extends EventEmitter {
         this._emitProjectRunStatus(
             this.threads.length + doneThreads.length -
                 this._getMonitorThreadCount([...this.threads, ...doneThreads]));
+        // Store threads that completed this iteration for testing and other
+        // internal purposes.
+        this._lastStepDoneThreads = doneThreads;
         if (this.renderer) {
             // @todo: Only render when this.redrawRequested or clones rendered.
             if (this.profiler !== null) {