fix coverage

src/main/java/com/github/bingoohuang/voucherno/Strs.java

@@ -2,7 +2,7 @@
 
 import java.util.stream.IntStream;
 
-public class Strs {
+public abstract class Strs {
     public static StringBuilder fixedLength(StringBuilder s, int len) {
         int diff = len - s.length();
         if (diff < 0) return s.delete(len, s.length());

src/test/java/com/github/bingoohuang/voucherno/StrsTest.java

@@ -0,0 +1,15 @@
+package com.github.bingoohuang.voucherno;
+
+
+import org.junit.Test;
+
+import static com.google.common.truth.Truth.assertThat;
+
+public class StrsTest {
+    @Test
+    public void fixedLength() {
+        assertThat(Strs.fixedLength(new StringBuilder("a"), 2).toString()).isEqualTo("a0");
+        assertThat(Strs.fixedLength(new StringBuilder("ab"), 2).toString()).isEqualTo("ab");
+        assertThat(Strs.fixedLength(new StringBuilder("abc"), 2).toString()).isEqualTo("ab");
+    }
+}