feat(module: modal): add modal test

[Guoyuanqiang]

feat(module: modal): add modal test

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?

[ ] Bugfix
[x] 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

[codecov]

Codecov Report

Merging #18 into master will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
+ Coverage   85.84%   85.93%   +0.08%     
==========================================
  Files         209      209              
  Lines        6875     6875              
  Branches      973      973              
==========================================
+ Hits         5902     5908       +6     
+ Misses        563      556       -7     
- Partials      410      411       +1

Impacted Files	Coverage Δ
components/modal/modal.component.ts	81.89% <0%> (+5.17%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ba9917d...31e873b. Read the comment docs.