fixes#24338
When unrolling each iteration of a `fields` iterator, the compiler only
opens a new scope for semchecking, but doesn't generate a node that
signals to the codegen that a new scope should be created. This causes
issues for reused template instantiations that reuse variable symbols
between each iteration, which causes the codegen to generate multiple
declarations for them in the same scope (regardless of `inject` or
`gensym`). To fix this, we wrap the unrolled iterations in an `if true:
body` node, which both opens a new scope and doesn't interfere with
`break`.
(cherry picked from commit ca5df9ab25)
* clean up some test categories
* mention exact slice issue
* magics into system
* move trangechecks into overflow
* move tmemory to system
* try fix CI
* try fix CI
* final CI fix
