tests: Skip if necessary ZIM file isn't present

The large ZIM files used for tests are not included in the release
tarball because of their size. But we still want the tests to pass
in that case, so skip if the file is missing.

Fixes #68.

tests/test_libzim_file_reader.py

@@ -26,6 +26,8 @@ def zimdata(request):
 
 @pytest.fixture
 def reader(zimdata):
+    if not zimdata["filename"].exists():
+        pytest.skip(f"{zimdata['filename']} doesn't exist")
     return File(zimdata["filename"])