Fix assertEquals in test_csv2cve.py

[terriko]

This is a very beginner-friendly bug suitable for someone who doesn't have any commits yet. Please don't bother to fix this one if you've already got commits -- it's really not urgent.

I basically made a typo in test_csv2cve that generates a warning in pytest:

================================================= warnings summary ==================================================
test/test_csv2cve.py::TestCsv2cve::test_main
  /home/terri/Code/cve-bin-tool/test/test_csv2cve.py:53: DeprecationWarning: Please use assertEqual instead.
    self.assertEquals(0, returncode)

-- Docs: https://docs.pytest.org/en/latest/warnings.html

----------- coverage: platform linux, python 3.6.8-final-0 -----------
Coverage XML written to file coverage.xml

===================================== 92 passed, 1 warning in 732.06s (0:12:12) =====================================

If you look at that warning, it pretty much tells you exactly what to do and where to find the issue! It should be a one line fix.