bug: Fix typo in testing_util/config.cmake.in

[ghost]

Sorry I didn't catch this earlier. Just caught it while trying to use find_package(google_cloud_cpp_testing CONFIG REQUIRED) in spanner.

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This change is 

[codecov]

Codecov Report

Merging #2851 into master will increase coverage by 0.26%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2851      +/-   ##
==========================================
+ Coverage   89.18%   89.44%   +0.26%     
==========================================
  Files         292      292              
  Lines       19519    19519              
==========================================
+ Hits        17408    17459      +51     
+ Misses       2111     2060      -51

Impacted Files	Coverage Δ
google/cloud/storage/oauth2/credentials.h	33.33% <0%> (-33.34%)	⬇️
google/cloud/bigtable/metadata_update_policy.h	71.42% <0%> (-28.58%)	⬇️
google/cloud/storage/well_known_headers.h	77.77% <0%> (-7.41%)	⬇️
google/cloud/storage/internal/generic_request.h	82.05% <0%> (ø)	⬆️
...storage/benchmarks/storage_throughput_benchmark.cc	82.15% <0%> (+0.41%)	⬆️
google/cloud/storage/object_stream.h	100% <0%> (+1.85%)	⬆️
google/cloud/bigtable/mutations.h	100% <0%> (+2.6%)	⬆️
google/cloud/storage/internal/object_requests.h	100% <0%> (+2.98%)	⬆️
google/cloud/storage/object_metadata.h	100% <0%> (+3.7%)	⬆️
google/cloud/storage/internal/curl_handle.h	77.41% <0%> (+12.9%)	⬆️
... and 3 more

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[coryan]

We are missing some kind of test to detect these problems methinks.

Reviewed 1 of 1 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved