Support mocking storage::Client.

[coryan]

We got this feature request from at least two users, recording it here.

Mocking most functions in this class is not hard:

google-cloud-cpp/google/cloud/storage/testing/mock_client.h

Line 27 in bcefc9f

class MockClient : public google::cloud::storage::internal::RawClient {

The hard thing is to mock Client::ReadObject() and Client::WriteObject().

• Mock typical unary operations.
• Mock ReadObject for "a read that works".
• Mock WriteObject for "a write that works".
• Mock ReadObject for a failure.
• Mock WriteObject for a failure.

[manish-qlogic]

example of mocking WriteStreamBuf

google-cloud-cpp/google/cloud/storage/client_write_object_test.cc

Line 60 in 0347fb5

class MockStreambuf : public internal::ObjectWriteStreambuf {

[coryan]

Yes, that is the one. For ObjectReadStreambuf you probably have to go one level deeper and mock the internal::ObjectReadSource class.

In both cases it would be nice to have an example that uses these mocks to read (or write) some data pre-defined by the user.

[manish-qlogic]

we will keep this task open as storage_client_mock_samples.cc need examples on failed scenarios.

google-cloud-cpp/google/cloud/storage/examples/storage_client_mock_samples.cc

Line 58 in 7f13737

void MockReadObject(int& argc, char* argv[]) {

[coryan]

I think this was fixed in #2924