test: convert os-release-tests to test-data

Use the new `locate_test_data()` helper to get access to test-data.
Guard the test with `have_test_data()` to skip it in case test-data
access is not available.

test/mod/test_util_osrelease.py

@@ -7,16 +7,19 @@ import unittest
 
 from osbuild.util import osrelease
 
+from .. import test
 
-class TestUtilOSRelease(unittest.TestCase):
+
+class TestUtilOSRelease(test.TestBase, unittest.TestCase):
     def test_non_existant(self):
         """Verify default os-release value, if no files are given."""
         self.assertEqual(osrelease.describe_os(), "linux")
 
+    @unittest.skipUnless(test.TestBase.have_test_data(), "no test-data access")
     def test_describe_os(self):
         """Test host os detection. test/os-release contains the os-release files
         for all supported runners.
         """
-        for entry in os.scandir("test/os-release"):
+        for entry in os.scandir(os.path.join(self.locate_test_data(), "os-release")):
             with self.subTest(entry.name):
                 self.assertEqual(osrelease.describe_os(entry.path), entry.name)