Make sure that all failure paths in new-sdist are tested

This one wasn't actually testing what it said it was testing. Oops.

cabal-testsuite/PackageTests/NewSdist/MultiTarget/valid-and-test-suite.out

 # cabal new-sdist
-cabal: The package a-test cannot be packaged for distribution, because it is not local to this project.
+cabal: The component test suite 'a-tests' cannot be packaged for distribution on its own. Only entire packages may be packaged for distribution.

cabal-testsuite/PackageTests/NewSdist/MultiTarget/valid-and-test-suite.test.hs

 import Test.Cabal.Prelude
 main = cabalTest $ withSourceCopy $ do
   cwd <- fmap testCurrentDir getTestEnv
-  fails $ cabal "new-sdist" ["a", "b", "a-test"]
+  fails $ cabal "new-sdist" ["a", "b", "a-tests"]
   shouldNotExist $ cwd </> "dist-newstyle/sdist/a-0.1.tar.gz"
   shouldNotExist $ cwd </> "dist-newstyle/sdist/b-0.1.tar.gz"