Increase unit test code coverage

[uniqueg]

According to the most recent code coverage report (on branch api-location in PR #46), there is still a bit of untested code:

Name              Stmts   Miss  Cover   Missing
-----------------------------------------------
tes/__init__.py       5      0   100%
tes/client.py       111      4    96%   133, 186-187, 211
tes/models.py       202     29    86%   29, 53, 65, 77, 87, 292, 296, 298, 300-301, 303-304, 306-307, 309-311, 320, 322, 324, 329, 331, 336-339, 342-344
tes/utils.py         57     11    81%   34-37, 44, 72, 81-86
-----------------------------------------------
TOTAL               375     44    88%

Increase code coverage by providing unit tests for the missing statements in:

• tes/client.py
• tes/models.py
• tes/utils.py