[antlir][image_test] Allow selecting run_as_user

Summary: In coverage mode, the user may need to be overridden to `root`.

Test Plan: CI

Reviewed By: vmagro

Differential Revision: D85779601

fbshipit-source-id: 7dc84d48352b2ee3df08e5edb16cb85244651556

Author: Anass Al-Wohoush

antlir/antlir2/testing/image_test.bzl

@@ -228,7 +228,7 @@ def _implicit_image_test(
         test_rule,
         name: str,
         layer: str | Select,
-        run_as_user: str | None = None,
+        run_as_user: str | Select | None = None,
         labels: list[str] | Select | None = None,
         boot: bool = False,
         boot_requires_units: [list[str], None] = None,