Add basic unit tests for parsing of command line verbosity flags

The test failed prior to the bug fix:

Unit Tests
  Distribution.Simple.Command
    option argument tests
      parses verbosity successfully:           OK
      handles argument parse error gracefully: FAIL
        message threw an exception: =2
        CallStack (from HasCallStack):
          error, called at src/Distribution/ReadE.hs:42:24 in Cabal-3.7.0.0-inplace:Distribution.ReadE

1 out of 2 tests failed (0.00s)

Cabal-tests/Cabal-tests.cabal

@@ -31,6 +31,7 @@ test-suite unit-tests
     UnitTests.Distribution.Compat.Time
     UnitTests.Distribution.Described
     UnitTests.Distribution.PkgconfigVersion
+    UnitTests.Distribution.Simple.Command
     UnitTests.Distribution.Simple.Glob
     UnitTests.Distribution.Simple.Program.GHC
     UnitTests.Distribution.Simple.Program.Internal

Cabal-tests/tests/UnitTests.hs

@@ -16,6 +16,7 @@ import Distribution.Compat.Time
 import qualified UnitTests.Distribution.Compat.CreatePipe
 import qualified UnitTests.Distribution.Compat.Time
 import qualified UnitTests.Distribution.Compat.Graph
+import qualified UnitTests.Distribution.Simple.Command
 import qualified UnitTests.Distribution.Simple.Glob
 import qualified UnitTests.Distribution.Simple.Program.GHC
 import qualified UnitTests.Distribution.Simple.Program.Internal
@@ -49,6 +50,8 @@ tests mtimeChangeCalibrated =
         (UnitTests.Distribution.Compat.Time.tests mtimeChange)
     , testGroup "Distribution.Compat.Graph"
         UnitTests.Distribution.Compat.Graph.tests
+    , testGroup "Distribution.Simple.Command"
+        UnitTests.Distribution.Simple.Command.tests
     , testGroup "Distribution.Simple.Glob"
         UnitTests.Distribution.Simple.Glob.tests
     , UnitTests.Distribution.Simple.Program.GHC.tests

Cabal-tests/tests/UnitTests/Distribution/Simple/Command.hs

+module UnitTests.Distribution.Simple.Command
+    ( tests
+    ) where
+
+import Distribution.Simple.Command
+import qualified Distribution.Simple.Flag as Flag
+import Distribution.Simple.Setup (optionVerbosity)
+import qualified Distribution.Verbosity as Verbosity
+import Test.Tasty
+import Test.Tasty.HUnit
+
+argumentTests :: [TestTree]
+argumentTests =
+  [ testCase "parses verbosity successfully" $ do
+      let p = commandParseArgs cmdUI isGlobal ["-v2"]
+      assertEqual "expected verbose" (Right verbose) $ evalParse p
+  , testCase "handles argument parse error gracefully" $ do
+      let p = commandParseArgs cmdUI isGlobal ["-v=2"]
+      assertEqual "expected error" (Left "errors") $ evalParse p
+  ]
+  where
+    -- evaluate command parse result, to force possible exceptions in 'f'
+    evalParse p = case p of
+      CommandErrors _         -> Left "errors"
+      CommandHelp _           -> Left "help"
+      CommandList _           -> Left "list"
+      CommandReadyToGo (f, _) -> Right $ f Flag.NoFlag
+    verbose = Flag.Flag Verbosity.verbose
+    isGlobal = True
+    cmdUI = CommandUI
+      { commandName = "cmd"
+      , commandSynopsis = "the command"
+      , commandUsage = \name -> name ++ " cmd -v[N]"
+      , commandDescription = Nothing
+      , commandNotes = Nothing
+      , commandDefaultFlags = Flag.NoFlag
+      , commandOptions = const [ optField ]
+      }
+    optField = optionVerbosity id const
+        
+tests :: [TestTree]
+tests =
+  [ testGroup "option argument tests" argumentTests
+  ]