Add the --no-magic flag to the .cabal file for the cabal-doctests test.

This appears to be necessary on OSX.

The --no-magic flag stops the doctest executable from expanding path
arguments, trying to locate the package db, etc.

This shouldn't be necessary with cabal-doctest, since all the
necessary options to pass to doctest are computed when running the Setup.hs
script.

See
#427 (comment).

Author: cdepillabout

test/cabal-doctests/cabal-doctests-test.cabal

@@ -19,6 +19,7 @@ library
   default-language: Haskell2010
 
 test-suite doctests
+  x-doctest-options: --no-magic
   type: exitcode-stdio-1.0
   main-is: Doctests.hs
   ghc-options: -threaded -rtsopts -with-rtsopts=-N