Client auth test failing on newer version of go

[kruskall]

APM Server version (apm-server version):

Description of the problem including expected versus actual behavior:

tests are failing on newer verions of go in main branch

Steps to reproduce:

Please include a minimal but complete recreation of the problem,
including server configuration, agent(s) used, etc. The easier you make it
for us to reproduce it, the more likely that somebody will take the time to
look at it.

Provide logs (if relevant):