Add internal/db/cassandra/conviction test

[kevindiu]

Description:

This PR creates internal/db/nocql/cassandra/conviction.go tests

Related Issue:

How Has This Been Tested?:

Environment:

• Go Version: 1.15.2
• Docker Version: 19.03.8
• Kubernetes Version: 1.18.2
• NGT Version: 1.12.1

Types of changes:

• Bug fix [type/bug]
• New feature [type/feature]
• Add tests [type/test]
• Security related changes [type/security]
• Add documents [type/documentation]
• Refactoring [type/refactoring]
• Update dependencies [type/dependency]
• Update benchmarks and performances [type/bench]
• Update CI [type/ci]

Changes to Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?
• Have you successfully ran tests with your changes locally?

Checklist:

• I have read the CONTRIBUTING document.
• I have checked open Pull Requests for the similar feature or fixes?
• I have added tests and benchmarks to cover my changes.
• I have ensured all new and existing tests passed.
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly.

[vdaas-ci]

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• ⏪ /rebase - rebase master

[codecov]

Codecov Report

Merging #799 into master will decrease coverage by 0.30%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #799      +/-   ##
==========================================
- Coverage   17.53%   17.23%   -0.31%     
==========================================
  Files         421      420       -1     
  Lines       19554    19500      -54     
==========================================
- Hits         3429     3361      -68     
- Misses      15915    15927      +12     
- Partials      210      212       +2     

Impacted Files	Coverage Δ
internal/db/nosql/cassandra/conviction.go	100.00% <100.00%> (+100.00%)	⬆️
internal/db/storage/blob/s3/reader/reader.go	65.90% <0.00%> (-31.82%)	⬇️
internal/errgroup/group.go	92.42% <0.00%> (-1.52%)	⬇️
internal/net/net.go
internal/worker/worker.go	83.33% <0.00%> (+1.04%)	⬆️
internal/worker/queue.go	98.33% <0.00%> (+3.33%)	⬆️

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[hlts2]

LGTM

[kpango]

LGTM