fix tests build failures with go1.15

[alessio]

Go1.15 has become stricter with conversions from string.
See https://golang.org/doc/go1.15#vet

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Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer
• Review Codecov Report in the comment section below once CI passes

[alessio]

@ethanfrey @clevinson please review

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (launchpad/backports@6a624c3). Click here to learn what that means.
The diff coverage is n/a.

@@                  Coverage Diff                   @@
##             launchpad/backports    #7073   +/-   ##
======================================================
  Coverage                       ?   51.51%           
======================================================
  Files                          ?      338           
  Lines                          ?    20585           
  Branches                       ?        0           
======================================================
  Hits                           ?    10605           
  Misses                         ?     9185           
  Partials                       ?      795           

[ethanfrey]

Looks good. And I learned that Sprint works without the trailing f