[enhancement] Conditional test skipping based on configured values (e.g. #[skip_if])

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

There is currently no declarative mechanism to skip `#[test]` functions or `#[test_proc]`s based on compile-time configuration values (`configured_value_or`). When running test suites across different build/parameter configurations, we want to selectively run only the tests relevant to that configuration:

```dslx
pub const COLOR = configured_value_or("color", Color::Red);

#[test_proc]
#[skip_if COLOR != Color::Red]
proc test_red { ... }
```

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

Currently, tests must inspect the configured value at runtime within the test body (e.g., within `next` for a proc) and wrap the execution in an `if` condition:

```dslx
next(state: ()) {
if COLOR == Color::Red {
// actual test logic
}
send(tok, terminator, true);
}
```

This harms readability and incurs runtime overhead from spawning, initializing, and stepping vacuous tests across configurations where they have nothing to test.

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

Provide an attribute or attribute argument (e.g., `#[skip_if ]` or `#[test_proc(skip_if = )]`) evaluated during elaboration/typechecking against configured constants.

If the condition evaluates to true, the test runner reports the test as skipped rather than elaborating and simulating a no-op test.

I believe in Rust, this is done through the `#[cfg(...)` attribute.

Start by tracing how DSLX #[test] and #[test_proc] attributes are handled during elaboration and typechecking, then inspect how the test runner reports results. Use configured_value_or and the proposed #[skip_if] condition as the behavioral examples; done means configured tests are skipped before elaboration and simulation while still being reported as skipped.

Written by the indexing model from the issue text.