update filewriter unit tests

[prjemian]

Once a new version of spec2nexus is released that has the fix for empty #O0 & #P0 lines, revise the tests/test_filewriter.py with these lines: https://github.com/prjemian/spec2nexus/blob/a4158a27fdcb64ea6700e62da7b41221057bcecd/tests/test_plugin.py#L200-L209

[prjemian]

Looks like:

    self.assertGreater(scan.header.raw.find("\n#O0 \n"), 0)
    self.assertGreater(scan.header.raw.find("\n#o0 \n"), 0)
    self.assertEqual(len(scan.header.O), 1)
    self.assertEqual(len(scan.header.O[0]), 0)
    self.assertEqual(len(scan.header.o), 1)
    self.assertEqual(len(scan.header.o[0]), 0)
    self.assertGreater(scan.raw.find("\n#P0 \n"), 0)
    self.assertEqual(len(scan.P), 1)
    self.assertEqual(len(scan.P[0]), 0)
    self.assertEqual(len(scan.positioner), 0)

[prjemian]

add to requirements.txt for spec2nexus >= 2021.1.2 (released just now)