Better corpus testing

[LilithHafner]

Whenever folks write new code in base or the stdlib that depends on JuliaSyntax bugfixes that have not been backported to flisp, CI will break on this repo. See #444, for an example where adding

https://github.com/JuliaLang/julia/blame/f2558c461c85be4220901f4c67cbce718ddc015b/test/core.jl#L8115

In JuliaLang/julia#52228 caused CI to fail here.

Short term solution: stop testing against test/core.jl
Long term solution: test against flisp using this corpus as of Julia v1.9 and do not include new code

[LilithHafner]

Another long term solution is to follow Julia's typo detection CI check: only flag parsing discrepancies that are not present in the previous commit of JuliaSyntax.

[LilithHafner]

I think that we should parse a massive corpus on the head and target commit of each PR and fail CI if there are any differences unless the PR is tagged with syntax change.