Split up syntax.jl tests in Base so that JuliaSyntax can test the non-controversial parts.

Right now, the `syntax.jl` tests are skipped (https://github.com/JuliaLang/JuliaSyntax.jl/blob/2e965a159dd9f87d216d2d50ecbd2ed4f9af2c5a/test/parse_packages.jl#L41-L46). However, this causes us to sometimes regress, see e.g. https://github.com/JuliaLang/julia/pull/57188/files#diff-266674e240feff7742a800c1c360befe28cd7a20f49dada34fef94a122acd90b where the test got changed and then caused regressions https://github.com/JuliaLang/julia/issues/57223.

It would probably be a good idea to split up `syntax.jl` into a "controversial" part which is not tested by JuliaSyntax and a standard part which does get tested. Ideally, as time moves on the controversial part is shrunk.