test/e2e: optimize e2e test time by replacing sleeps with TCP readiness checks (#5223)

Replace the fixed 500ms sleep after each frps startup in RunProcesses
with a TCP dial-based readiness check that polls the server bind port.
This reduces the e2e suite wall time from ~97s to ~43s.

Also simplify the RunProcesses API to accept a single server template
string instead of a slice, matching how every call site uses it.