[enhancement] Support barriers in testing/eval Procs

### What's hard to do? (limit 100 words)

Currently procs can block for multiple reasons. For XLS's interpreters' purposes, a Tick() can return once procs are blocked and progress is made.

https://github.com/google/xls/blob/1578b44caac9cc63fc96924f09197d1d03dcd64c/xls/interpreter/proc_runtime.h#L119-L129

This issue is to enhance
1. XLS runtimes to support tick'ing until a set or procs are AtStartOfTick() and hold them.
2. DSLX language enhancements to support a barrier-like instruction so that a test proc can rely on `#1000`

Related:
https://github.com/google/xls/issues/1538
https://github.com/google/xls/issues/1527

### Current best alternative workaround (limit 100 words)

None

### Your view of the "best case XLS enhancement" (limit 100 words)

DSLX should have a barrier or a WaitAndHoldProcsAtStart() built-in.