Flesh out bonding fixtures

As part of change prometheus/procfs#439, update fixtures in
node_exporter to expose all referenced interfaces in /sys/class/net

This was naively accomplished by blindly copying eth0 data to other
physical interfaces, but that does not appear to have resulted in any
failing tests.  Once we begin to actually export bonding/LACP metrics,
additional modification of fixtures may be required.

Signed-off-by: Brandon Ewing <[email protected]>