internal/https: wait for both handlers in ListenAndServe
internal/https: wait for both handlers in ListenAndServe

Fixes golang/go#29941

Change-Id: I15a50983bb8c52b4f55bfbfb4ebe5aee1b10c73a
Reviewed-on: https://go-review.googlesource.com/c/build/+/159701
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build