replace testutil.NewTestCaseDir() with Go1.15's T.TempDir()

[alessio]

Override go's default version with go 1.15.

---

Before we can merge this PR, please make sure that all the following items have been
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• 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
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• Review Codecov Report in the comment section below once CI passes

[tac0turtle]

I'd wait a week before making this change. go 1.15 came out yesterday so many toole have not been updated yet

[tac0turtle]

this should be good to go, @alessio i updated to master

[tac0turtle]

merp, go 1.15 is available but not being used. hmmmmmm

[codecov]

Codecov Report

Merging #7014 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7014      +/-   ##
==========================================
- Coverage   54.86%   54.86%   -0.01%     
==========================================
  Files         586      586              
  Lines       35961    35958       -3     
==========================================
- Hits        19731    19728       -3     
  Misses      14310    14310              
  Partials     1920     1920              

[amaury1093]

Nice!