chore: update conformance test

[dopiera]

This fixes #3943.

Update to commit
googleapis/conformance-tests@dfdfff5

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This change is 

[coryan]

Reviewed 3 of 3 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved

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google/cloud/storage/tests/signed_url_conformance_test.cc, line 120 at r1 (raw file):

  }
  std::string const date = j_obj["timestamp"];
  auto const valid_for = std::chrono::seconds(j_obj["expiration"].get<int>());

FYI, there is a helper function (internal::ParseIntField()) that parses JSON fields that can be an int or a std::string with a number inside.

[codecov]

Codecov Report

Merging #3980 into master will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3980      +/-   ##
==========================================
+ Coverage   93.89%   93.92%   +0.03%     
==========================================
  Files         645      645              
  Lines       48815    48821       +6     
==========================================
+ Hits        45833    45856      +23     
+ Misses       2982     2965      -17     

Impacted Files	Coverage Δ
.../cloud/storage/client_sign_policy_document_test.cc	98.86% <ø> (ø)
...d/storage/internal/policy_document_request_test.cc	100.00% <ø> (ø)
.../cloud/storage/internal/policy_document_request.cc	98.61% <100.00%> (+0.02%)	⬆️
...cloud/storage/tests/signed_url_conformance_test.cc	79.81% <100.00%> (ø)
google/cloud/storage/upload_options.h	83.33% <0.00%> (-16.67%)	⬇️
google/cloud/storage/testing/random_names.cc	95.83% <0.00%> (-4.17%)	⬇️
google/cloud/bigtable/mutations.h	100.00% <0.00%> (ø)
google/cloud/bigquery/read_result.h	0.00% <0.00%> (ø)
google/cloud/internal/retry_policy.h	100.00% <0.00%> (ø)
google/cloud/storage/client_options.h	100.00% <0.00%> (ø)
... and 15 more

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[dopiera]

Reviewable status: 2 of 5 files reviewed, all discussions resolved (waiting on @coryan)

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google/cloud/storage/tests/signed_url_conformance_test.cc, line 120 at r1 (raw file):

Previously, coryan (Carlos O'Ryan) wrote…

FYI, there is a helper function (internal::ParseIntField()) that parses JSON fields that can be an int or a std::string with a number inside.

I didn't notice this, thanks. Change for mor futureproofness.

[coryan]

Reviewed 3 of 3 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved