Improve test coverage

[erlend-aasland]

I'd like to raise the test suite cover more of the code base, as a part of the preparations for #358.

Currently, coverage.py reports 73% code coverage when run locally:

Name                                    Stmts   Miss  Cover
-----------------------------------------------------------
canopen/__init__.py                        12      2    83%
canopen/_version.py                        11      2    82%
canopen/emcy.py                            71     19    73%
canopen/lss.py                            181    106    41%
canopen/network.py                        165     21    87%
canopen/nmt.py                            133      2    98%
canopen/node/__init__.py                    2      0   100%
canopen/node/base.py                        9      0   100%
canopen/node/local.py                      81     10    88%
canopen/node/remote.py                     81     37    54%
canopen/objectdictionary/__init__.py      306     37    88%
canopen/objectdictionary/datatypes.py      82      8    90%
canopen/objectdictionary/eds.py           349     38    89%
canopen/objectdictionary/epf.py            81     81     0%
canopen/pdo/__init__.py                    36      8    78%
canopen/pdo/base.py                       374    108    71%
canopen/profiles/__init__.py                0      0   100%
canopen/profiles/p402.py                  277    176    36%
canopen/sdo/__init__.py                     5      0   100%
canopen/sdo/base.py                        93     20    78%
canopen/sdo/client.py                     461     92    80%
canopen/sdo/constants.py                   27      0   100%
canopen/sdo/exceptions.py                  14      5    64%
canopen/sdo/server.py                     127      9    93%
canopen/sync.py                            19      7    63%
canopen/timestamp.py                       15      0   100%
canopen/utils.py                           13      1    92%
canopen/variable.py                        98     42    57%
-----------------------------------------------------------
TOTAL                                    3123    831    73%

73% is very promising; getting up to 80% is the next milestone.

In #504, I raised the coverage of canopen/nmt.py to 98%. I'm going to continue with canopen/network.py (anticipate PR in a day or two). Feel free to join!

I'll update this list as progress is made:

• canopen/emcy.py: WIP
• canopen/lss.py
• canopen/network.py:
  - Tests: improve coverage of Network #515
  - Tests: improve coverage of NodeScanner #517
• canopen/nmt.py: Add tests for NMT base and NMT master #504
• canopen/node/base.py
• canopen/node/local.py
• canopen/node/remote.py
• canopen/objectdictionary/__init__.py
• canopen/objectdictionary/datatypes.py
• canopen/objectdictionary/eds.py
• canopen/objectdictionary/epf.py
• canopen/pdo/__init__.py
• canopen/pdo/base.py
• canopen/profiles/p402.py
• canopen/sdo/base.py
• canopen/sdo/client.py
• canopen/sdo/constants.py
• canopen/sdo/exceptions.py
• canopen/sdo/server.py
• canopen/sync.py: WIP
• canopen/timestamp.py
• canopen/utils.py
• canopen/variable.py