Fix skipUnless and factorize

Author: hsyl20

cabal-testsuite/PackageTests/JS/JsSources/cabal.test.hs

@@ -2,7 +2,7 @@ import Test.Cabal.Prelude
 
 main = setupAndCabalTest $ do
     skipUnlessGhcVersion ">= 9.6"
-    skipUnless =<< isJavaScript
+    skipUnlessJavaScript
 
     res <- cabal' "v2-run" ["demo"]
     assertOutputContains "Hello JS!" res

cabal-testsuite/src/Test/Cabal/Prelude.hs

@@ -889,6 +889,12 @@ skipUnlessGhcVersion range = skipUnless ("needs ghc " ++ range) =<< isGhcVersion
 skipIfGhcVersion :: String -> TestM ()
 skipIfGhcVersion range = skipUnless ("incompatible with ghc " ++ range) =<< isGhcVersion range
 
+skipUnlessJavaScript :: TestM ()
+skipUnlessJavaScript = skipUnless "needs the JavaScript backend" =<< isJavaScript
+
+skipIfJavaScript :: TestM ()
+skipIfJavaScript = skipIf "incompatible with the JavaScript backend" =<< isJavaScript
+
 isWindows :: TestM Bool
 isWindows = return (buildOS == Windows)