Add test for duplicate environment variables when invoking testsuite

Adds a simple test case that identifies and reports duplicate
environment variables in the Cabal environment.

For issue (#10718)

cabal-testsuite/PackageTests/DuplicateEnvVars/cabal.project

+packages: p

cabal-testsuite/PackageTests/DuplicateEnvVars/cabal.test.hs

+import Test.Cabal.Prelude
+
+main = cabalTest $ recordMode DoNotRecord $ do
+    res <- cabal' "test" ["all"]
+    assertOutputContains "No duplicate environment variables found" res

cabal-testsuite/PackageTests/DuplicateEnvVars/p/Main.hs

+module Main where
+
+import Data.List (group, sort)
+import System.Environment (getEnvironment)
+
+main = do
+    env <- getEnvironment
+    let sortedEnv = sort env
+        duplicates = filter (\g -> length g > 1) $ group $ map fst sortedEnv
+
+    if null duplicates
+        then putStrLn "No duplicate environment variables found."
+        else do
+            putStrLn "Found duplicate environment variables:"
+            mapM_ (\d -> putStrLn $ "  - " ++ head d) duplicates
+            fail "Test failed due to duplicate environment variables"

cabal-testsuite/PackageTests/DuplicateEnvVars/p/p.cabal

+cabal-version:      3.0
+name:               p
+version:            0.1.0.0
+build-type:         Simple
+
+test-suite env-test
+    default-language: Haskell2010
+    type:             exitcode-stdio-1.0
+    main-is:          Main.hs
+    build-depends:    base