[ET][arm] Fix broken internal arm tests

## Context

Fix test failures caused by #9081 (D70889669). Apply suggested fix from @hsz.

Also addresses T168807700

Differential Revision: [D70979587](https://our.internmc.facebook.com/intern/diff/D70979587/)

[ghstack-poisoned]

Author: SS-JIA

backends/arm/test/conftest.py

@@ -32,7 +32,7 @@ def pytest_configure(config):
     pytest._test_options = {}  # type: ignore[attr-defined]
     pytest._test_options["corstone_fvp"] = False  # type: ignore[attr-defined]
 
-    if config.option.arm_run_corstoneFVP:
+    if getattr(config.option, "arm_run_corstoneFVP", False) and config.option.arm_run_corstoneFVP:
         corstone300_exists = shutil.which("FVP_Corstone_SSE-300_Ethos-U55")
         corstone320_exists = shutil.which("FVP_Corstone_SSE-320")
         if not (corstone300_exists and corstone320_exists):