[antlir2][image_test] expose inner_test as subtarget

Summary:
This makes it very easy to find the path of the test and run it from a shell in
the container.

Test Plan: testhard

Reviewed By: justintrudell

Differential Revision: D50176038

fbshipit-source-id: d0ca12df955cefbb5871dd0c0fd4fe5649dea796

antlir/antlir2/testing/image_test.bzl

@@ -71,7 +71,10 @@ def _impl(ctx: AnalysisContext) -> list[Provider]:
         RunInfo(test_cmd),
         DefaultInfo(
             script,
-            sub_targets = {"layer": ctx.attrs.layer.providers},
+            sub_targets = {
+                "inner_test": ctx.attrs.test.providers,
+                "layer": ctx.attrs.layer.providers,
+            },
         ),
     ]