add test for File class

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")