Add test for Armstrong (#3104)

Co-authored-by: Andrii Siriak <siryaka@gmail.com>

src/test/java/com/thealgorithms/maths/TestArmstrong.java

@@ -0,0 +1,15 @@
+package com.thealgorithms.maths;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+public class TestArmstrong {
+
+    @Test
+    public void testArmstrong() {
+        Armstrong armstrong = new Armstrong();
+        assertThat(armstrong.isArmstrong(371)).isTrue();
+        assertThat(armstrong.isArmstrong(200)).isFalse();
+    }
+}