tests: fixture records with reference text and identifier

[ptamarit]

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Fixes https://github.com/zenodo/ops/issues/415

Description

The test in test_marcxml_serializer.py has an assertion on 999C5$x which make it look as if the reference identifier was serialized instead of the reference text. This is due to the fact that the fake test data did not follow the real format (with a reference text, an identifier and a scheme).

This pull request makes the test data for references match with the reality and modifies affected assertions.

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