The google.cloud.auth package provides a Credentials class for use with
emulators (and other systems that do not require oauth credentials). Use
this rather than rolling out own.

It seems a couple of people around the internet have copied this method
for running code against the emulators. We should ensure that if people
are going to copy these tests, that at least they do something less
hacky.

We remove the requirement for the GOOGLE_APPLICATION_CREDENTIALS if we
detect that we should be running against the local datastore emulator.

This unit test was overwriting the datastore client being used to be a
default constructed client. This was causing it to be constructed with
the default credentials object, which requires that the test is either
run in GCP or with the `GOOGLE_APPLICATION_CREDENTIALS` environment
variable set.

Move away from assertTrue to a helper that outputs the actual value and
the expected value when the test fails.

The last test run in TestDatastoreQuery would fail as it was trying to
remove more than 500 entities at once. We break the deletes up into
batches to work around this limitation of the datastore api.

These tests assume that specific pieces of data exist within datastore,
but nothing in the test created these pieces of data.

Also, the TestDatastoreQuery.test_query_offset_timestamp_keys test
assumes there is >= 10k entities in the datastore, but the create
function would only add 1k entities.

The datastore emulator does not check the size of the response messages
before trying to send them back to the client. For all of the tests that
return large objects, this results in the response exceeding the ~4MiB
grpc message limit (enforces in message_size_filter). The response size
for these tests is ~10MiB.

Since we do not need the actual contents of the entity, we can request
just the keys, via a keys_only query.

Technically this is changing they type of query, which may be considered
testing a different thing. I will leave this up to the maintainers if
they want to merge this change.

There is both a 500 entities per mutation limit, and a ~4MiB
request/response size limit. Update both the populate and clear
libraries to stay below these limits.