Fix pytest --generate --fixtures flag

[jamesharris-garmin]

This patch resolves the issues where pytest will try to double load the
pytest_cmd_main.

This resolves issue #204

[jamesharris-garmin]

@youtux you might find this useful.

[coveralls]

Coverage remained the same at 96.114% when pulling 2d4485e on jamesharris-garmin:fix-generation-command into 386ed90 on pytest-dev:master.

[youtux]

Thanks for the fix. Can you also add these 2 lines at the end of test_generate_missing so that we cover it for the future?

assert not result.stderr.str()
assert result.ret == 0

[jamesharris-garmin]

Done and done!

[youtux]

Thanks for the fix!