Add coverage for query offset fix

[tseaver]

/cc @crwilcox

PR #18 did not add coverage for the case it was fixing.

nox > Running session cover
nox > Re-using existing virtual environment at .nox/cover.
nox > python -m pip install coverage pytest-cov
nox > coverage report --show-missing --fail-under=97
Name                                                                                  Stmts   Miss Branch BrPart  Cover   Missing
---------------------------------------------------------------------------------------------------------------------------------
google/cloud/datastore/__init__.py                                                        9      0      0      0   100%
google/cloud/datastore/_app_engine_key_pb2.py                                            23      0      2      0   100%
google/cloud/datastore/_gapic.py                                                         15      0      2      0   100%
google/cloud/datastore/_http.py                                                          72      0     12      0   100%
google/cloud/datastore/batch.py                                                         106      0     32      0   100%
google/cloud/datastore/client.py                                                        230      0    118      0   100%
google/cloud/datastore/entity.py                                                         23      0      6      0   100%
google/cloud/datastore/helpers.py                                                       206      0    132      0   100%
google/cloud/datastore/key.py                                                           202      0     76      0   100%
google/cloud/datastore/query.py                                                         202      3     64      1    98%   588->596, 596-598
...

[craiglabenz]

I had been wondering about this. To state the obvious, CI is not currently blocking on test coverage below 100%. Is this intentional?

[tseaver]

@craiglabenz We've been below 100% since the microgenerator merge back in October 2020. I consider it a fairly big process flaw, and fixing this bit is the last "manual" gap (the others are all in autogenerated async client code, see googleapis/gapic-generator-python#632).