Add default-language pragmas to cabal-testsuite.cabal

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

cabal-testsuite/cabal-testsuite.cabal

@@ -46,6 +46,7 @@ library
     build-depends: unix, exceptions
   else
     build-depends: Win32
+  default-language: Haskell2010
 
 test-suite cabal-tests
   type: exitcode-stdio-1.0
@@ -61,6 +62,7 @@ test-suite cabal-tests
     process,
     optparse-applicative,
     cabal-testsuite
+  default-language: Haskell2010
 
 custom-setup
   setup-depends: Cabal >= 1.25,