add test for File class

tests/test_file.py

+import unittest
+from csst_common import File
+
+
+class TestFile(unittest.TestCase):
+    def test_parameterized_module_decorator(self):
+        f = File("/path/to/file.fits")
+        self.assertEqual(f.switch_extension(".cat"), "/path/to/file.cat")
+        self.assertEqual(f.switch_extension("img.cat"), "/path/to/file_img.cat")