### What this PR does / why we need it?
This PR adds mooncake common method to conftest, we need it to add more
test cases later
### Does this PR introduce _any_ user-facing change?
No
### How was this patch tested?
by running a test
- vLLM version: v0.14.0
- vLLM main:
d68209402d
Signed-off-by: jiangyunfan1 <jiangyunfan1@h-partners.com>