make GetUnusedPort concurrency safe

[muff1nman]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Kwok can be used nicely in go test with parallelism. However when running with -race, GetUnusedPort is flagged as a data race. Ancedotally, during our tests we have seen port contention and this might help resolve that.

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.: