fix(module:textarea): fix textarea test case error

[sWhite01111]

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-mobile/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?

[x] 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
[x] No

Other information

[codecov]

Codecov Report

Merging #36 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master     #36   +/-   ##
======================================
  Coverage    85.9%   85.9%           
======================================
  Files         209     209           
  Lines        6889    6889           
  Branches      977     977           
======================================
  Hits         5918    5918           
  Misses        559     559           
  Partials      412     412

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