Add test: RTS warning

cabal-testsuite/PackageTests/NewBuild/CmdRun/WarningRTS/Main.hs

+main = putStrLn "Hello World"
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cabal-testsuite/PackageTests/NewBuild/CmdRun/WarningRTS/WarningRTS.cabal

+name: WarningRTS
+version: 1.0
+build-type: Simple
+cabal-version: >= 1.10
+
+executable foo
+    main-is: Main.hs
+    build-depends: base
+    default-language: Haskell2010
+
+library
+    exposed-modules: Main
+    default-language: Haskell2010

cabal-testsuite/PackageTests/NewBuild/CmdRun/WarningRTS/cabal.project

+packages: .

cabal-testsuite/PackageTests/NewBuild/CmdRun/WarningRTS/cabal.test.hs

+import Test.Cabal.Prelude
+
+main = cabalTest $ do
+    -- your test code here
+    res <- cabal' "run" ["+RTS"]
+    assertOutputContains       "Warning" res
+
+    res <- cabal' "run" [ "+RTS", "--"]
+    assertOutputContains       "Warning" res
+
+    res <- cabal' "run" [ "--", "+RTS"]
+    assertOutputDoesNotContain "Warning" res
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