Port custom-setup/new_build_requires_Cabal_1_20 to new test suite.

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

cabal-install/tests/IntegrationTests/custom-setup/new_build_requires_Cabal_1_20.sh

-# Regression test for issue #3932
-
-. ./common.sh
-
-cd custom-setup-old-cabal
-! cabal new-build > output 2>&1
-
-cat output
-grep -q "(issue #3932) requires >=1.20" output || die "Expect constraint failure"

cabal-testsuite/PackageTests/Regression/T3932/cabal.project

+packages: .

cabal-testsuite/PackageTests/Regression/T3932/cabal.test.hs

+import Test.Cabal.Prelude
+main = cabalTest $
+    -- This repository contains a Cabal-1.18.0.0 option, which would
+    -- normally would satisfy the repository, except for new-build's
+    -- extra constraint that setup Cabal must be 1.20.  If we don't
+    -- have a choice like this available, the unsatisfied constraint
+    -- won't be reported.
+    withRepo "repo" $ do
+        fails (cabal' "new-build" []) >>=
+            assertOutputContains "(issue #3932) requires >=1.20"

cabal-testsuite/PackageTests/Regression/T3932/repo/Cabal-1.18.0.0/Cabal.cabal

+name: Cabal
+version: 1.18.0.0
+build-type: Simple
+cabal-version: >= 1.10
+
+library

cabal-testsuite/Test/Cabal/Prelude.hs

@@ -96,6 +96,10 @@ withDirectory f = withReaderT
 withEnv :: [(String, Maybe String)] -> TestM a -> TestM a
 withEnv e = withReaderT (\env -> env { testEnvironment = testEnvironment env ++ e })
 
+-- HACK please don't use me
+withEnvFilter :: (String -> Bool) -> TestM a -> TestM a
+withEnvFilter p = withReaderT (\env -> env { testEnvironment = filter (p . fst) (testEnvironment env) })
+
 ------------------------------------------------------------------------
 -- * Running Setup