I understand you are trying to test E2E.

If you are using bean of the client directly, It needs to be separate server and client. Then you need to bind actual port.

In this case, the following guide would be best as i know.

https://gist.github.com/silkentrance/b5adb52d943555671a44e88356c889f8

Or you can just fix boot port but parallel test case execution is prohibited.

If you do not need to use the client interface directly, You can just MVC or REST client test for your server instead of feign client. or controller execution directly. You are losing good point of feign but do not need to consider the boot process for port binding during test.