add crossref test case

[pep8speaks]

Hello @njzjz! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file tests/cases.py:

Line 114:80: E501 line too long (134 > 79 characters) Line 123:80: E501 line too long (98 > 79 characters) Line 125:80: E501 line too long (98 > 79 characters)

• In the file tests/test_bibtex.py:

Line 16:80: E501 line too long (84 > 79 characters)

• In the file tests/test_cli.py:

Line 16:80: E501 line too long (80 > 79 characters) Line 26:80: E501 line too long (88 > 79 characters)

• In the file tests/test_from_identifier.py:

Line 14:80: E501 line too long (81 > 79 characters)