Make it easier to run the tests

You can now run a test in the nixos/tests directory directly using
nix-build, e.g.

  $ nix-build '<nixos/tests/login.nix>' -A test

This gets rid of having to add the test to nixos/tests/default.nix.
(Of course, you still need to add it to nixos/release.nix if you want
Hydra to run the test.)

nixos/lib/testing.nix

@@ -73,7 +73,7 @@ rec {
   apply = makeTest; # compatibility
   call = f: f { inherit pkgs system; };
 
-  complete = { testScript, ... } @ t: t // rec {
+  complete = { testScript, makeCoverageReport ? false, ... } @ t: t // rec {
 
     nodes = buildVirtualNetwork (
       t.nodes or (if t ? machine then { machine = t.machine; } else { }));
@@ -117,6 +117,8 @@ rec {
     test = runTests driver;
 
     report = releaseTools.gcovReport { coverageRuns = [ test ]; };
+
+    result = if makeCoverageReport then report else test;
   };