(cabal check) Add "No `main-is`" test

cabal-testsuite/PackageTests/Check/ConfiguredPackage/Sanity/NoMainIs/cabal.out

+# cabal check
+Warning: The package will not build sanely due to these errors:
+Warning: No 'main-is' field found for executable exe
+Warning: Hackage would reject this package.

cabal-testsuite/PackageTests/Check/ConfiguredPackage/Sanity/NoMainIs/cabal.test.hs

+import Test.Cabal.Prelude
+
+-- Missing `main-is`.
+main = cabalTest $
+  fails $ cabal "check" []

cabal-testsuite/PackageTests/Check/ConfiguredPackage/Sanity/NoMainIs/pkg.cabal

+cabal-version: 3.0
+name: pkg
+synopsis: synopsis
+description: description
+version: 0
+category: example
+maintainer: none@example.com
+license: GPL-3.0-or-later
+
+executable exe
+    default-language: Haskell2010