net/netutil: add test coverage for ipForwardingEnabledLinux per-interface reads

The existing test only exercised the not-found-interface path. Now that
ipForwardingEnabledLinux opens its sysctl key with os.OpenInRoot
(840c6e3d3, #20572), also verify that the global keys and the
per-interface keys for every interface actually present on the machine
can be read without error, for both IPv4 and IPv6.

Updates #20572

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: Ie204a163ab9f8670abedd79a4ac81e400f71aab7