go1.22: Remove loop variable lexical rebindings after a future gopls update

[pgporada]

There is a bug in gopls that requires us to rebind the loop variable if/when we run subtests in parallel such as what I did in https://github.com/letsencrypt/boulder/pull/7438 and https://github.com/letsencrypt/boulder/pull/7452.

Once a newer version of gopls has been released and our tooling begins using it, let's remove occurrences of testCases := testCases, tc:=tc, etc.