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).