Add --tasty-arg to validate.sh

This lets you add custom arguments to `Tasty` test suites. This can be
used to set `--keep-tmp-files` for debugging, or to run a particular
test in `cabal-testsuite`:

    ./validate.sh -v -s build -s cli-suite \
        --tasty-arg PackageTests/HaddockKeepTmpsCustom/cabal.test.hs

cabal-validate/src/Cli.hs

@@ -234,9 +234,11 @@ resolveOpts opts = do
 
  tastyArgs' =
  "--hide-successes"
- : case rawTastyPattern opts of
- Just tastyPattern -> ["--pattern", tastyPattern]
- Nothing -> []
+ : maybe
+ []
+ (\tastyPattern -> ["--pattern", tastyPattern])
+ (rawTastyPattern opts)
+ ++ rawTastyArgs opts
 
  when (rawListSteps opts) $ do
  -- TODO: This should probably list _all_ available steps, not just the selected ones!
@@ -279,6 +281,7 @@ data RawOpts = RawOpts
  , rawCabal :: FilePath
  , rawExtraCompilers :: [FilePath]
  , rawTastyPattern :: Maybe String
+ , rawTastyArgs :: [String]
  , rawDoctest :: Bool
  , rawSteps :: [Step]
  , rawListSteps :: Bool
@@ -343,6 +346,12 @@ rawOptsParser =
  <> help "Pattern to filter tests by"
  <> value Nothing
  )
+ <*> many
+ ( strOption
+ ( long "tasty-arg"
+ <> help "Extra arguments to pass to Tasty test suites"
+ )
+ )
  <*> boolOption
  False
  "doctest"