nettest, *: add option to run HTTP tests with in-memory network

To avoid ephemeral port / TIME_WAIT exhaustion with high --count
values, and to eventually detect leaked connections in tests. (Later
the memory network will register a Cleanup on the TB to verify that
everything's been shut down)

Updates tailscale/corp#27636

Change-Id: Id06f1ae750d8719c5a75d871654574a8226d2733
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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