[enhancement] DSLX tests should be able to set/view proc state

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

In writing DSLX tests, it is desirable to

Inspect the state of an individual proc so that architectural state can be observed

Inject/set the proc state so that the initial state can be set without going through a long input sequence to get to the desired state.

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

Current workaround is to write the test in C++ which has access to the state

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

Provide functionality akin to:

```
proc a_test {
config ( ...) {
let X = spawn A();
let Y = spawn B();
}

next () {
// < run test>
assert_eq(X.state.val0, Y.state.val1);
}
}
```