feat(file-cleaner): added unit tests

file-cleaner/src/test/java/de/begerad/fileagerm/TestFileCleaner.java

@@ -0,0 +1,4 @@
+package de.begerad.fileagerm;
+
+public class TestFileCleaner {
+}

file-cleaner/src/test/java/de/begerad/fileagerm/TestFileExplorer.java

@@ -0,0 +1,21 @@
+package de.begerad.fileagerm;
+
+import org.junit.jupiter.api.Test;
+
+import java.io.File;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class TestFileExplorer {
+
+    @Test
+    public void testEmptyListFiles() {
+        File[] files = FileExplorer.listFiles("/mnt");
+        assertTrue(files.length == 0);
+    }
+
+    @Test
+    public void testNullListFiles() {
+        assertNull(FileExplorer.listFiles("/etc/hosts"));
+    }
+}