Run test discovery locally to save RE resources

Summary:
Static test listing (discovery) currently runs on Remote Execution workers even though it doesn't need to. This wastes RE resources that could be used for actual test execution. This change makes the Buck2 test orchestrator automatically select the "listing" executor override for discovery stages, so that listing runs locally when the target has `local_listing_enabled` set in its `remote_execution` dict.

The Buck2 prelude already creates a "listing" executor override with `local_enabled = True` when this field is present, but nothing was selecting it. Rather than round-tripping through labels and having tpx send the override name back, the orchestrator now directly checks for a "listing" executor override during listing stages and falls back to the default executor if none exists.

**CONTEXT**

S610793 is a sample sev caused by lack of quota for RE compute on 'some operation'(multisect). The operation here was simply listing, which can be done on local linux machine. The aim of this rollout is to optimize RE usage for test runs by shifting the listing workload for RE tests with the static listing setup to local. We expect all targets to be compatible with this.

Reviewed By: patskovn

Differential Revision: D92956543

fbshipit-source-id: 6938511df9776441ded1f8a54ee8cb5589e366a3

Author: yahayaohinoyi

app/buck2_build_api/src/interpreter/rule_defs/provider/builtin/external_runner_test_info.rs

@@ -155,6 +155,10 @@ impl FrozenExternalRunnerTestInfo {
         unpack_opt_executor(self.default_executor.get().to_value()).unwrap()
     }
 
+    pub fn has_executor_overrides(&self) -> bool {
+        !self.executor_overrides.get().to_value().is_none()
+    }
+
     /// Access a specific executor override.
     pub fn executor_override(&self, key: &str) -> Option<&StarlarkCommandExecutorConfig> {
         let executor_overrides =

app/buck2_test/src/orchestrator.rs

@@ -1356,7 +1356,13 @@ impl BuckTestOrchestrator<'_> {
                     })?
                     .0,
             ),
-            None => test_info.default_executor().map(|o| &o.0),
+            None => match stage {
+                TestStage::Listing { .. } if test_info.has_executor_overrides() => test_info
+                    .executor_override("listing")
+                    .or(test_info.default_executor())
+                    .map(|o| &o.0),
+                _ => test_info.default_executor().map(|o| &o.0),
+            },
         };
 
         let executor_config = Self::executor_config_with_remote_cache_override(