fix: fixed the Lesson15Test.java file

Author: haldanek

lesson_15/tdd/tdd_app/src/test/java/com/codedifferently/lesson15/Lesson15Test.java

@@ -1 +1,19 @@
+package com.codedifferently.lesson15;
 
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.jupiter.api.Test;
+
+class Lesson15Test {
+
+  @Test
+  public void testLesson15() {
+    assertThat(new Lesson15()).isNotNull();
+  }
+
+  @Test
+  public void testGetGreeting() {
+    // Act
+    Lesson15.main(null);
+  }
+}