test: add core case and coverage ignore file

[shuaijinchao]

No description provided.

[codecov]

Codecov Report

Merging #23 (a0959a8) into master (62a68f2) will increase coverage by 1.33%.
The diff coverage is 90.00%.

❗ Current head a0959a8 differs from pull request most recent head acee2d4. Consider uploading reports for the commit acee2d4 to get more accurate results

@@            Coverage Diff             @@
##           master      #23      +/-   ##
==========================================
+ Coverage   84.06%   85.40%   +1.33%     
==========================================
  Files          13       13              
  Lines         634      637       +3     
==========================================
+ Hits          533      544      +11     
+ Misses        101       93       -8     

Impacted Files	Coverage Δ
apisix/runner/plugin/core.py	96.66% <85.71%> (+30.00%)	⬆️
apisix/runner/http/response.py	91.33% <100.00%> (ø)
apisix/runner/server/response.py	100.00% <100.00%> (ø)

---

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 62a68f2...acee2d4. Read the comment docs.