Fix bug which prevented the static cached config from being used

Summary:
JustKnobs provides two ways to `init` with a `cached_config`:
* [`init_just_knobs`](https://www.internalfb.com/intern/rustdoc/fbcode/common/rust/shed/justknobs_stub:justknobs/justknobs/cached_config/fn.init_just_knobs.html)
* [`init_just_knobs_worker`](https://www.internalfb.com/intern/rustdoc/fbcode/common/rust/shed/justknobs_stub:justknobs/justknobs/cached_config/fn.init_just_knobs_worker.html)

The former is simple and stores the provided config as a static `HashMap` of knobs.
The latter is fancier: given a tokio runtime, it starts a thread that watches a file for changes.

The former is good enough for test situations where we just want to parse some json and run tests with it.
But... it is unusable (prior to this change) as for a static config, we never declare it to be "`in_use`", which means that when we run `eval`, we fall back to the actual prod jk despite initializing the simple static justknobs.

Not anymore.

Reviewed By: YousefSalama

Differential Revision: D59867073

fbshipit-source-id: aaeb0d94b158c840ba34b34c15ac050fe4885205

shed/justknobs_stub/src/cached_config.rs

@@ -54,7 +54,7 @@ enum KnobVal {
 }
 
 pub fn in_use() -> bool {
-    JUST_KNOBS_WORKER_STATE.get().is_some()
+    JUST_KNOBS_WORKER_STATE.get().is_some() || JUST_KNOBS.get().is_some()
 }
 
 pub fn just_knobs() -> &'static ArcSwap<JustKnobsInMemory> {
@@ -245,11 +245,7 @@ mod test {
         init_just_knobs(&logger, &config_handle)?;
         assert!(CachedConfigJustKnobs::eval("my/config:knob1", None, None).unwrap());
 
-        // This is a problem: the public interface for interacting with this jk errs, despite being
-        // initialized with a static config handle that contains this justknob.
-        assert!(justknobs::eval("my/config:knob1", None, None).is_err());
-        // We should expect this to work:
-        //     assert!(justknobs::eval("my/config:knob1", None, None).unwrap());
+        assert!(justknobs::eval("my/config:knob1", None, None).unwrap());
         Ok(())
     }
 }