Test that the last line `--help` is the name of the config file

Ensure that the last line of the help text is the name of the config file.
This invariant is used by clients such as the Haskell setup github action.

See: https://github.com/haskell-actions/setup/pull/63

cabal-testsuite/PackageTests/Help/HelpPrintsConfigFile/help-prints-config-file.out

+# cabal --help

cabal-testsuite/PackageTests/Help/HelpPrintsConfigFile/help-prints-config-file.test.hs

+-- Andreas Abel, 2024-01-13
+--
+-- Ensure that the last line of the help text is the name of the config file.
+-- This invariant is used by clients such as the Haskell setup github action.
+-- See: https://github.com/haskell-actions/setup/pull/63
+
+import Distribution.Utils.String (trim)
+import Test.Cabal.Prelude
+
+main = cabalTest $ do
+  env <- getTestEnv
+  res <- cabal' "--help" []
+
+  -- The end of the help text should be something like:
+  --
+  -- > You can edit the cabal configuration file to set defaults:
+  -- >   <<HOME>>/.cabal/config
+  --
+  -- So trimming the last line will give us the name of the config file.
+  let configFile = trim . last . lines . resultOutput $ res
+
+  -- Verify that this is indeed the config file.
+  assertEqual "Last line of help text should be name of the config file"
+    (testUserCabalConfigFile env)
+    configFile