We want to be able to use the netx.Network (and RealNetwork
implemementation) outside of tests, without linking "testing".
So split out the non-test stuff of nettest into its own package.
We tend to use "foox" as the convention for things we wish were in the
standard library's foo package, so "netx" seems consistent.
Updates tailscale/corp#27636
Change-Id: I1911d361f4fbdf189837bf629a20f2ebfa863c44
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
03b47a55c7