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

[KristofferC]

Right now, the syntax.jl tests are skipped (

JuliaSyntax.jl/test/parse_packages.jl

Lines 41 to 46 in 2e965a1

	# syntax.jl has some intentionally weird syntax which we parse
	# differently than the flisp parser, and some cases which we've
	# decided are syntax errors.
	if endswith(f, "syntax.jl")
	return nothing
	end

). 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 JuliaLang/julia#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.