Add a test to check that extra-prog-path is honoured for local packages

Whilst fixing haskell#10692, I realised there was also this bug where
extra-prog-path would not be honoured for specific packages.

The idea behind extra-prog-path is that each local package can use a
different version of a preprocessor if desired.

cabal-testsuite/PackageTests/ExtraProgPathLocal/cabal.project

@@ -0,0 +1 @@
+packages: client

cabal-testsuite/PackageTests/ExtraProgPathLocal/client/Hello.x

@@ -0,0 +1,3 @@
+module Main where
+
+main = print 0

cabal-testsuite/PackageTests/ExtraProgPathLocal/client/client.cabal

@@ -0,0 +1,12 @@
+name:                client
+version:             0.1.0.0
+synopsis:            Checks build-tool-depends are put in PATH
+license:             BSD3
+category:            Testing
+build-type:          Simple
+cabal-version:       >=1.10
+
+executable             hello-world
+  main-is:             Hello.hs
+  build-depends:       base
+  default-language:    Haskell2010

cabal-testsuite/PackageTests/ExtraProgPathLocal/scripts/alex

@@ -0,0 +1,3 @@
+#! /usr/bin/env bash
+
+echo "I am not the alex you are looking for"

cabal-testsuite/PackageTests/ExtraProgPathLocal/scripts2/alex

@@ -0,0 +1,4 @@
+#! /usr/bin/env bash
+
+echo "I am the alex you are looking for"
+echo "module Main where main = print ()" > $3

cabal-testsuite/PackageTests/ExtraProgPathLocal/setup.out

@@ -0,0 +1,10 @@
+# cabal v2-build
+Configuration is affected by the following files:
+- cabal.project
+Resolving dependencies...
+Build profile: -w ghc-<GHCVER> -O1
+In order, the following will be built:
+ - client-0.1.0.0 (exe:hello-world) (first run)
+Configuring executable 'hello-world' for client-0.1.0.0...
+Preprocessing executable 'hello-world' for client-0.1.0.0...
+Building executable 'hello-world' for client-0.1.0.0...

cabal-testsuite/PackageTests/ExtraProgPathLocal/setup.test.hs

@@ -0,0 +1,7 @@
+import Test.Cabal.Prelude
+-- Test package local extra-prog-path works.
+main = cabalTest $ do
+    skipIfWindows
+    env <- getTestEnv
+    liftIO $ appendFile (testCurrentDir env </> "cabal.project") $ "\npackage client\n  extra-prog-path:" ++ (testCurrentDir env </> "scripts2/")
+    addToPath (testTmpDir env </> "scripts/") $ cabal "v2-build" ["client"]