Applies assertion funcs to cache tests

[rkapka]

What type of PR is this?

Other/Tests Refactoring

What does this PR do? Why is it needed?

applies testutil/assert and testutil/require to cache tests

Which issues(s) does this PR fix?

Part of #6590

Other notes for review

I don't know how to assert that a slice variable is nil. Trying assert.Equal(t, ([]unit64)(nil), x) results in

panic: runtime error: comparing uncomparable type []uint64 [recovered]
        panic: runtime error: comparing uncomparable type []uint64

[codecov]

Codecov Report

Merging #6617 into master will decrease coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6617      +/-   ##
==========================================
- Coverage   61.50%   61.42%   -0.09%     
==========================================
  Files         375      373       -2     
  Lines       29298    29237      -61     
==========================================
- Hits        18021    17958      -63     
- Misses       8819     8820       +1     
- Partials     2458     2459       +1     

[farazdagi]

Looks good, thank you!