(cabal check) Add "Version signatures" test

`signatures` field can be used only with `cabal-version` ≥ 2.0

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

+# cabal check
+Warning: These warnings may cause trouble when distributing the package:
+Warning: pkg.cabal:17:3: The field "signatures" is available only since the Cabal specification version 2.0. This field will be ignored.

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

+import Test.Cabal.Prelude
+
+-- `signatures` field used with cabal-version < 2.0
+main = cabalTest $
+  fails $ cabal "check" []

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

+cabal-version: 1.24
+build-type: Custom
+name: pkg
+synopsis: synopsis
+description: description
+version: 0
+category: example
+maintainer: none@example.com
+license: GPL-3
+license-file: LICENSE
+
+custom-setup
+
+library
+  exposed-modules: Foo
+  default-language: Haskell2010
+  signatures: Str