Add expectBroken test for #7423

cabal-testsuite/PackageTests/Check/NoWarnFlag/cabal.out

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+# cabal check
+No errors or warnings could be found in the package.

cabal-testsuite/PackageTests/Check/NoWarnFlag/cabal.test.hs

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+import Test.Cabal.Prelude
+
+-- Do not output warning when an -O2 is behind a cabal flag.
+main = cabalTest . expectBroken 7423 $ do
+         res <- cabal' "check" []
+         assertOutputContains "No errors or warnings could be found in the package." res
+
+       -- once the test is not broken, replace main with:
+       -- main = cabalTest $ cabal "check" []
+

cabal-testsuite/PackageTests/Check/NoWarnFlag/pkg.cabal

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+cabal-version: 2.2
+name: pkg
+version: 0
+category: example
+maintainer: [email protected]
+synopsis: synopsys
+description: description
+license: GPL-3.0-or-later
+
+flag force-O2
+  default: False
+  manual: True
+
+library
+  exposed-modules:      Foo
+  default-language:     Haskell2010
+  if flag(force-O2)
+    ghc-options:        -O2