test/e2e: optimize RunFrps/RunFrpc with process exit detection (#5225)

* test/e2e: optimize RunFrps/RunFrpc with process exit detection

Refactor Process to track subprocess lifecycle via a done channel,
replacing direct cmd.Wait() in Stop() to avoid double-Wait races.
RunFrps/RunFrpc now use select on the done channel instead of fixed
sleeps, allowing short-lived processes (verify, startup failures) to
return immediately while preserving existing timeout behavior for
long-running daemons.

* test/e2e: guard Process against double-Start and Stop-before-Start

Add started flag to prevent double-Start panics and allow Stop to
return immediately when the process was never started. Use sync.Once
for closing the done channel as defense-in-depth against double close.