Add tests for latest function

tests/test_core.py

 #!/usr/bin/env python3
 # -*- coding: utf-8 -*-
+import os
 import unittest
 
-from km3net_testdata import data_path
+from km3net_testdata import data_path, latest, LATEST_VERSIONS
 
 
 class TestDataPath(unittest.TestCase):
     def test_access(self):
-        assert data_path("km3net_offline.root").endswith("km3net_offline.root")
+        assert data_path("km3net_online.root").endswith("km3net_online.root")
+
+    def test_accessing_nonexistent_files(self):
+        with self.assertRaises(RuntimeError):
+            data_path("nonexistent_file")
+
+    def test_filename_returned_if_raise_missing_is_false(self):
+        filename = "nonexistent_file"
+        assert data_path(filename, raise_missing=False).endswith(filename)
+
+
+class TestLatest(unittest.TestCase):
+    def test_latest_paths(self):
+        for dataformat in LATEST_VERSIONS.keys():
+            assert latest(dataformat).endswith(LATEST_VERSIONS[dataformat])