fix(pubsub): install admin mocks

[dbolduc]

Part of the work for #5782

We were copying all of the mock files into both include/google/cloud/pubsub/mocks and include/google/cloud/pubsub/admin/mocks, without preserving the relative directory structure.

The handwritten and generated admin mock files have the same name, so I think we would first install the generated admin mocks (because they were first in the list), then we would overwrite them with the handwritten admin mocks.

Before:

docker:fedora-latest-cmake$ ls /h/google-cloud-cpp-installed/include/google/cloud/pubsub/mocks
mock_ack_handler.h		      mock_publisher_connection.h  mock_subscriber_connection.h
mock_blocking_publisher_connection.h  mock_pull_ack_handler.h	   mock_subscription_admin_connection.h
mock_exactly_once_ack_handler.h       mock_schema_connection.h	   mock_topic_admin_connection.h
docker:fedora-latest-cmake$ head -1 /h/google-cloud-cpp-installed/include/google/cloud/pubsub/mocks/mock_topic_admin_connection.h 
// Copyright 2020 Google LLC

docker:fedora-latest-cmake$ ls /h/google-cloud-cpp-installed/include/google/cloud/pubsub/admin/mocks
mock_ack_handler.h		      mock_publisher_connection.h  mock_subscriber_connection.h
mock_blocking_publisher_connection.h  mock_pull_ack_handler.h	   mock_subscription_admin_connection.h
mock_exactly_once_ack_handler.h       mock_schema_connection.h	   mock_topic_admin_connection.h
docker:fedora-latest-cmake$ head -1 /h/google-cloud-cpp-installed/include/google/cloud/pubsub/admin/mocks/mock_topic_admin_connection.h 
// Copyright 2020 Google LLC

After:

docker:fedora-latest-cmake$ ls /h/google-cloud-cpp-installed/include/google/cloud/pubsub/mocks
mock_ack_handler.h		      mock_publisher_connection.h  mock_subscriber_connection.h
mock_blocking_publisher_connection.h  mock_pull_ack_handler.h	   mock_subscription_admin_connection.h
mock_exactly_once_ack_handler.h       mock_schema_connection.h	   mock_topic_admin_connection.h
docker:fedora-latest-cmake$ head -1 /h/google-cloud-cpp-installed/include/google/cloud/pubsub/mocks/mock_topic_admin_connection.h 
// Copyright 2020 Google LLC

docker:fedora-latest-cmake$ ls /h/google-cloud-cpp-installed/include/google/cloud/pubsub/admin/mocks
mock_subscription_admin_connection.h  mock_topic_admin_connection.h
docker:fedora-latest-cmake$ head -1 /h/google-cloud-cpp-installed/include/google/cloud/pubsub/admin/mocks/mock_topic_admin_connection.h 
// Copyright 2023 Google LLC

---

Note that we are going to need to use the $<BUILD_INTERFACE:...> generator expression in order to install our other mocks too.

---

This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (115a348) 93.14% compared to head (9b20939) 93.14%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #13537   +/-   ##
=======================================
  Coverage   93.14%   93.14%           
=======================================
  Files        2192     2192           
  Lines      190738   190738           
=======================================
+ Hits       177660   177663    +3     
+ Misses      13078    13075    -3     

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