test(module:button): added missing tests

[ParsaArvanehPA]

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: missing unit tests for the button Component

What is the current behavior?

What is the new behavior?

	Before	After
Statements	100%	100%
Branches	93.33%	100%
Functions	100%	100%
Lines	100%	100%

Does this PR introduce a breaking change?

• [ ] Yes
• [✔] No

Other information