tests: disable fsmonitor.fallback-on-watchman-exception

Summary: This can cover up unexpected bugs/crashes in Watchman integration, so don't fall back for tests.

Reviewed By: quark-zju

Differential Revision: D51265960

fbshipit-source-id: cd97cc38e5a9a5ee333aff5b18cb9dd2cb779298

eden/scm/tests/default_hgrc.py

@@ -68,6 +68,7 @@ def get_content(use_watchman: bool = False, use_ipv6: bool = False) -> str:
 
 [fsmonitor]
 detectrace=True
+fallback-on-watchman-exception=false
 """
 
     return content