Fix `chgrp` test failure

[forsyth2]

testChgrpHPSS fails with AssertionError: Command=`hsi ls -l zstash_test`. Errors=['This was supposed to be found, but was not: 3.', 'This was supposed to be found, but was not: m.']. This occurs as far as back as v1.0.0 -- I'm unsure why this is happening, since the unit tests have been run successfully multiple time since then.

In tests/test_chgrp.py, expected_present is a string, but should be a list.

                expected_present = 'e3sm'
            else:
                expected_present = 'chgrp: HPSS is unavailable'
            self.check_strings(cmd, output + err, expected_present, ['ERROR'])

Note, however, that even changing it to a list does not make the test pass.

[forsyth2]

Partially fixed by #179.