ci: split t/plugin to reduce CI run time

[soulbird]

Description

Currently the t/plugin test case takes too long to run, causing the entire CI to take too long to run.
Split the t/plugin test case into two parts and execute them concurrently to reduce the overall running time of CI.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[tzssangglass]

CI / build (ubuntu-20.04, linux_openresty, t/plugin/[a-o]*) (pull_request) Successful in 45m
CI / build (ubuntu-20.04, linux_openresty, t/plugin/[p-z]*) (pull_request) Successful in 33m

CI Centos7 / run ci on centos7 (t/plugin/[a-o]*) (pull_request) Successful in 43m
CI Centos7 / run ci on centos7 (t/plugin/[p-z]*) (pull_request) Successful in 33m

It looks like [a-o] is taking ten more minutes