test: reduce integration test flakiness

[amchiclet]

The test now retries when connecting to the metadata server.

Go 1.17 build constraint syntax was also added.

Tested by temporarily having presubmit also run the backoff tests.

The Golint check may fail because there is no .mod file. This is intentional and integration_test.sh does the module initialization.

Windows check may also fail but can be ignored (Windows is not supported).

[codecov]

Codecov Report

Merging #807 (c47b84c) into main (4cc74b7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #807   +/-   ##
=======================================
  Coverage   69.55%   69.55%           
=======================================
  Files           7        7           
  Lines        1248     1248           
  Branches       53       53           
=======================================
  Hits          868      868           
  Misses        379      379           
  Partials        1        1           

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