refactor: revise obsolete solution logic

Solve the problem in less than 80 characters.

Author: ParanoidUser

kata/8-kyu/up-and-down-the-string-grows/main/StrangeString.java

@@ -1,5 +1,3 @@
-class StrangeString {
-  static final String STRANGE_STRING = "ß";
-
-  private StrangeString() {}
+interface StrangeString {
+  String STRANGE_STRING = "ß";
 }

kata/8-kyu/up-and-down-the-string-grows/test/SolutionTest.java

@@ -1,5 +1,9 @@
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
 import org.junit.jupiter.api.Test;
 
 class SolutionTest {
@@ -8,4 +12,11 @@ void sample() {
     String upDown = StrangeString.STRANGE_STRING.toUpperCase().toLowerCase();
     assertEquals(StrangeString.STRANGE_STRING.length() + 1, upDown.length());
   }
+
+  @Test
+  void solutionSize() throws Exception {
+    Path current = Paths.get("").toAbsolutePath();
+    Path solution = current.resolve("main/StrangeString.java");
+    assertTrue(Files.size(solution) < 81, "Solution should be 80 characters max.");
+  }
 }