tests: skip unrar test gracefully if module is missing

Fedora (and probably some other distributions) cannot include unrardll
because of licensing reasons. Unfortuantely this situation is unlikely
to change in the near future. Skip the test if the module is not
available.

src/calibre/test_build.py

@@ -17,6 +17,10 @@
 
 is_ci = os.environ.get('CI', '').lower() == 'true'
 
+try:
+    import unrardll
+except ModuleNotFoundError:
+    unrardll = None
 
 class BuildTest(unittest.TestCase):
 
@@ -236,6 +240,7 @@ def test_file_dialog_helper(self):
         from calibre.gui2.win_file_dialogs import test
         test()
 
+    @unittest.skipUnless(unrardll, 'Module unrardll is missing')
     def test_unrar(self):
         from calibre.utils.unrar import test_basic
         test_basic()