cleanup(pubsublite): fix integration test resource cleanup

[dbolduc]

In #9678, I forgot to set topic_name_ upon initialization, so we were not deleting the topic in the test's destructor. Oops, my bad. https://github.com/googleapis/google-cloud-cpp/blob/ad5acebb8a413baeafbb2fb9857fc2086215795e/google/cloud/pubsublite/integration_tests/publisher_integration_test.cc#L85

I noticed that most leaked topics were getting deleted, though. Turns out we will make a request like projects/cloud-cpp-testing-resources/locations/..., but receive a response like: projects/123456789/locations/..... So the string comparison to check whether a topic was stale was always coming up true. Fix this by only comparing the topic id.

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This change is 

[google-cloud-cpp-bot]

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[codecov[bot]]

Codecov Report

Merging #9685 (42ee627) into main (ad5aceb) will increase coverage by 0.00%. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #9685   +/-   ##
=======================================
  Coverage   94.29%   94.29%           
=======================================
  Files        1494     1494           
  Lines      139063   139063           
=======================================
+ Hits       131124   131126    +2     
+ Misses       7939     7937    -2     

Impacted Files	Coverage Δ
...le/cloud/internal/default_completion_queue_impl.cc	96.59% <0.00%> (-0.57%)	:arrow_down:
google/cloud/pubsub/samples/samples.cc	90.77% <0.00%> (+0.07%)	:arrow_up:
...cloud/pubsub/internal/subscription_session_test.cc	98.15% <0.00%> (+0.33%)	:arrow_up:

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