Add check tests for default-language (#6288)

Cabal/Cabal.cabal

Cabal/Distribution/FieldGrammar/Newtypes.hs

Cabal/tests/CheckTests.hs

Cabal/tests/ParserTests/regressions/issue-6288-a.cabal

Cabal/tests/ParserTests/regressions/issue-6288-a.check

Cabal/tests/ParserTests/regressions/issue-6288-b.cabal

Cabal/tests/ParserTests/regressions/issue-6288-b.check

Cabal/tests/ParserTests/regressions/issue-6288-c.cabal

Cabal/tests/ParserTests/regressions/issue-6288-c.check

Cabal/tests/ParserTests/regressions/issue-6288-d.cabal

Cabal/tests/ParserTests/regressions/issue-6288-d.check

Cabal/tests/ParserTests/regressions/issue-6288-e.cabal

Cabal/tests/ParserTests/regressions/issue-6288-e.check

Cabal/tests/ParserTests/regressions/issue-6288-f.cabal

Cabal/tests/ParserTests/regressions/issue-6288-f.check