Fix TestBase example in kdoc

Author: mareklangiewicz

kotlinx-coroutines-core/jvm/test/TestBase.kt

@@ -31,14 +31,14 @@ public actual val stressTestMultiplier = stressTestMultiplierSqrt * stressTestMu
  * thread can be written. Use it like this:
  *
  * ```
- * class MyTest {
+ * class MyTest : TestBase() {
  *    @Test
- *    fun testSomething() = runBlocking<Unit> { // run in the context of the main thread
+ *    fun testSomething() = runBlocking { // run in the context of the main thread
  *        expect(1) // initiate action counter
- *        val job = launch(context) { // use the context of the main thread
+ *        launch { // use the context of the main thread
  *           expect(3) // the body of this coroutine in going to be executed in the 3rd step
  *        }
- *        expect(2) // launch just scheduled coroutine for exectuion later, so this line is executed second
+ *        expect(2) // launch just scheduled coroutine for execution later, so this line is executed second
  *        yield() // yield main thread to the launched job
  *        finish(4) // fourth step is the last one. `finish` must be invoked or test fails
  *    }