* Handle "listen hungup" error on Plan 9 in TestHttpsInsecure

Plan 9 returns the "listen hungup" error when closing a closed listener.

See https://github.com/0intro/plan9/blob/4225c3e/sys/src/9/ip/devip.c#L476

Fixes #253.

* Handle empty profile on Plan 9 in TestHttpsInsecure

CPU profiling isn't implemented on Plan 9.

See golang.org/issues/22564.

Fixes #253.