pkgs/build-support/cabal: add support for running specific tests only via 'testTarget'

The dns packages requires this feature, because it ships two test programs: one
of them requires network access (so we cannot run it), but the other test does
not. Setting testTarget appropriately allows us to run only one of the two
suites.

pkgs/build-support/cabal/default.nix

@@ -80,6 +80,9 @@ assert enableCheckPhase -> stdenv.lib.versionOlder "7" ghc.ghcVersion;
             # build-depends Cabal fields stated in test-suite stanzas
             testDepends = [];
 
+            # target(s) passed to the cabal test phase as an argument
+            testTarget = "";
+
             # build-tools Cabal field
             buildTools = [];
 
@@ -159,7 +162,7 @@ assert enableCheckPhase -> stdenv.lib.versionOlder "7" ghc.ghcVersion;
             checkPhase = stdenv.lib.optional self.doCheck ''
               eval "$preCheck"
 
-              ./Setup test
+              ./Setup test ${self.testTarget}
 
               eval "$postCheck"
             '';