test: add e2e test cases for cors plugin

[tokers]

Please answer these questions before submitting a pull request

• Why submit this pull request?

• Bugfix

• New feature provided

• Improve performance

• Backport patches

• Related issues

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Bugfix

• Description

• How to fix?

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New feature or improvement

• Describe the details and related test reports.

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Backport patches

• Why need to backport?

• Source branch

• Related commits and pull requests

• Target branch

[codecov-io]

Codecov Report

Merging #334 (90fb180) into master (8006297) will decrease coverage by 0.48%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #334      +/-   ##
==========================================
- Coverage   43.72%   43.24%   -0.49%     
==========================================
  Files          40       39       -1     
  Lines        3428     3427       -1     
==========================================
- Hits         1499     1482      -17     
- Misses       1761     1775      +14     
- Partials      168      170       +2     

Impacted Files	Coverage Δ
pkg/apisix/cluster.go	28.44% <0.00%> (-5.97%)	⬇️
pkg/apisix/route.go	42.03% <0.00%> (-1.92%)	⬇️
test/e2e/e2e.go

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