Use t.TempDir() in tests

[farazdagi]

What type of PR is this?

Other

What does this PR do? Why is it needed?

• As of Go 1.15 support for temporary directories has been added (see t.TempDir()):

TempDir returns a temporary directory for the test to use. The directory is automatically removed by Cleanup when the test and all its subtests complete. Each subsequent call to t.TempDir returns a unique directory; if the directory creation fails, TempDir terminates the test by calling Fatal.

• In several places we have number of different implementation allowing us to have a temporary directory, which is unique and removed after the test. All this is handled by the new t.TempDir().

Which issues(s) does this PR fix?

N/A

Other notes for review

[codecov]

Codecov Report

Merging #7769 (303c4ad) into master (7f7d18e) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #7769      +/-   ##
==========================================
- Coverage   62.14%   62.12%   -0.02%     
==========================================
  Files         429      429              
  Lines       30370    30362       -8     
==========================================
- Hits        18873    18863      -10     
- Misses       8556     8561       +5     
+ Partials     2941     2938       -3