Cleanup abstractions of "TestCase" class in unittest

tests/FactorGraphs/ScheduleStepTest.php

@@ -0,0 +1,17 @@
+<?php
+
+namespace DNW\Skills\Tests\FactorGraphs;
+
+use DNW\Skills\FactorGraphs\ScheduleStep;
+use DNW\Skills\FactorGraphs\Factor;
+use PHPUnit\Framework\TestCase;
+
+class ScheduleStepTest extends TestCase
+{
+    public function test(): void
+    {
+        $stub = $this->createStub(Factor::class);
+        $ss = new ScheduleStep('dummy', $stub, 0);
+        $this->assertEquals('dummy', (string)$ss);
+    }
+}