Add an intercepting cabal-testsuite/cabal.project

- Reduce duplication in test flag setup
- Avoid "New config" picking cabal's own project

Before this change, this was the test failure:

Unit Tests
  UnitTests.Distribution.Client.Configure
    Configure tests
      New config:                     FAIL
        Exception: project-cabal/pkgs/cabal.config: withBinaryFile: does not exist (No such file or directory)
        Use -p '/New config/' to rerun this test only.
      Replacement + new config:       OK
      Old + new config:               OK
      Old + new config, no appending: OK
      Old + new config, backup check: OK
      Local program options:          OK

1 out of 6 tests failed (0.02s)

cabal-install/tests/UnitTests/Distribution/Client/Configure.hs

@@ -23,13 +23,22 @@ tests =
   [ configureTests
   ]
 
+defaultTestFlags :: NixStyleFlags ()
+defaultTestFlags =
+  (defaultNixStyleFlags ())
+    { projectFlags =
+        mempty
+          { flagProjectDir = Flag projectDir
+          }
+    }
+
 configureTests :: TestTree
 configureTests =
   testGroup
     "Configure tests"
     [ testCase "New config" $ do
         let flags =
-              (defaultNixStyleFlags ())
+              defaultTestFlags
                 { configFlags =
                     mempty
                       { configOptimization = Flag MaximumOptimisation
@@ -42,7 +51,7 @@ configureTests =
           @=? (packageConfigOptimization . projectConfigLocalPackages $ snd projConfig)
     , testCase "Replacement + new config" $ do
         let flags =
-              (defaultNixStyleFlags ())
+              defaultTestFlags
                 { configExFlags =
                     mempty
                       { configAppend = Flag True
@@ -52,10 +61,6 @@ configureTests =
                       { configOptimization = Flag NoOptimisation
                       , configVerbosity = Flag silent
                       }
-                , projectFlags =
-                    mempty
-                      { flagProjectDir = Flag projectDir
-                      }
                 }
         (_, ProjectConfig{..}) <- configureAction' flags [] defaultGlobalFlags
 
@@ -63,7 +68,7 @@ configureTests =
         Flag silent @=? projectConfigVerbosity projectConfigBuildOnly
     , testCase "Old + new config" $ do
         let flags =
-              (defaultNixStyleFlags ())
+              defaultTestFlags
                 { configExFlags =
                     mempty
                       { configAppend = Flag True
@@ -72,10 +77,6 @@ configureTests =
                     mempty
                       { configVerbosity = Flag silent
                       }
-                , projectFlags =
-                    mempty
-                      { flagProjectDir = Flag projectDir
-                      }
                 }
         (_, ProjectConfig{..}) <- configureAction' flags [] defaultGlobalFlags
 
@@ -83,15 +84,11 @@ configureTests =
         Flag silent @=? projectConfigVerbosity projectConfigBuildOnly
     , testCase "Old + new config, no appending" $ do
         let flags =
-              (defaultNixStyleFlags ())
+              defaultTestFlags
                 { configFlags =
                     mempty
                       { configVerbosity = Flag silent
                       }
-                , projectFlags =
-                    mempty
-                      { flagProjectDir = Flag projectDir
-                      }
                 }
         (_, ProjectConfig{..}) <- configureAction' flags [] defaultGlobalFlags
 
@@ -99,15 +96,11 @@ configureTests =
         Flag silent @=? projectConfigVerbosity projectConfigBuildOnly
     , testCase "Old + new config, backup check" $ do
         let flags =
-              (defaultNixStyleFlags ())
+              defaultTestFlags
                 { configFlags =
                     mempty
                       { configVerbosity = Flag silent
                       }
-                , projectFlags =
-                    mempty
-                      { flagProjectDir = Flag projectDir
-                      }
                 }
             backup = projectDir </> "cabal.project.local~"
 
@@ -122,16 +115,12 @@ configureTests =
     , testCase "Local program options" $ do
         let ghcFlags = ["-fno-full-laziness"]
             flags =
-              (defaultNixStyleFlags ())
+              defaultTestFlags
                 { configFlags =
                     mempty
                       { configVerbosity = Flag silent
                       , configProgramArgs = [("ghc", ghcFlags)]
                       }
-                , projectFlags =
-                    mempty
-                      { flagProjectDir = Flag projectDir
-                      }
                 }
         (_, ProjectConfig{..}) <- configureAction' flags [] defaultGlobalFlags
 

cabal-testsuite/cabal.project

+-- This intercepting project is here to avoid tests picking up a cabal.project
+-- from a parent directory, such as the one in the root of the `haskell/cabal`
+-- project itself. Having `optional-packages: .` avoids the folowing warning
+-- being added to the `.out` file:
+--   Warning: There are no packages or optional-packages in the project
+optional-packages: .