ci: test one package at a time

[tsahee]

This averts a bug where gotesum seems to succeed if there were failures in multiple packages, some packages were o.k. after rerunning failures and some were not.

not sure if this is the best way - but it seems to work.

Here is an example of the problem: see how Go Test(default) step passes CI, and it's "run tests without race detection" step appears as passing even though it did record errors:
https://github.com/OffchainLabs/nitro/actions/runs/10015286108/job/27686456877

Here is execution for the same ci with said fix - notice that both (defaults) and (race) fail - correctly, when executed 3 times:
https://github.com/OffchainLabs/nitro/actions/runs/10014238547?pr=2504

[PlasmaPower]

I think we should write a shell script, perhaps in .github/workflows or perhaps written to disk in a previous ci step, that does the for package in go list ./...; ... work and the stdbuf work to just accept a list of gotestsum options. That said, I'd also be fine merging this as-is, given that it fixes an issue that otherwise allows CI to pass for failing tests.

[PlasmaPower]

LGTM for now, though I'd still like to refactor this soonish