fix: enable issue of endpointslices for k8s discovery

[slayer321]

Description

use conf.watch_endpoint_slices to enable endpointslices whenever it is set to true. Fixed the test.

Fixes #11631

Checklist

• [x] I have explained the need for this PR and the problem it solves
• [x] I have explained the changes or the new features added to this PR
• [x] I have added tests corresponding to this change
• [x] 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)