nixos/tests/predictable-interface-names: Refactor

The Nix expression here is really hard to read with multiple (and
unnecessarily) nested lets and it also generates attribute names based
on the derivation generated by makeTest, which will result in these
attribute names:

  * vm-test-run-predictableInterfaceNames
  * vm-test-run-predictableInterfaceNames-with-networkd
  * vm-test-run-unpredictableInterfaceNames
  * vm-test-run-unpredictableInterfaceNames-with-networkd

With the refactor the attribute names are now:

  * predictable
  * predictableNetworkd
  * unpredictable
  * unpredictableNetworkd

So now the code is even shorter and IMHO slightly more readable.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @symphorien, @fpletz, @adisbladis