feat(module:cron-expression): add Unit Testing

[OriginRing]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information

[zorro-bot]

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[codecov]

Codecov Report

Merging #7993 (a5478a1) into master (6d31bde) will decrease coverage by 0.11%.
The diff coverage is 100.00%.

❗ Current head a5478a1 differs from pull request most recent head 37d5670. Consider uploading reports for the commit 37d5670 to get more accurate results

@@            Coverage Diff             @@
##           master    #7993      +/-   ##
==========================================
- Coverage   91.80%   91.69%   -0.11%     
==========================================
  Files         509      514       +5     
  Lines       17452    17573     +121     
  Branches     2774     2690      -84     
==========================================
+ Hits        16022    16114      +92     
- Misses       1136     1164      +28     
- Partials      294      295       +1     

Impacted Files	Coverage Δ
...nents/cron-expression/cron-expression.component.ts	77.01% <100.00%> (ø)

... and 6 files with indirect coverage changes