remotefilelog: tweak corrupt cache test to grep more flexibly

I'm about to fix the abuse of RuntimeError here, which breaks the test
if I don't do this.

Differential Revision: https://phab.mercurial-scm.org/D11005

tests/test-remotefilelog-corrupt-cache.t

@@ -37,7 +37,7 b' Verify corrupt cache error message'
   > EOF
   $ chmod u+w $CACHEDIR/master/11/f6ad8ec52a2984abaafd7c3b516503785c2072/1406e74118627694268417491f018a4a883152f0
   $ echo x > $CACHEDIR/master/11/f6ad8ec52a2984abaafd7c3b516503785c2072/1406e74118627694268417491f018a4a883152f0
-  $ hg up tip 2>&1 | egrep "^RuntimeError"
+  $ hg up tip 2>&1 | egrep "^[^ ].*unexpected remotefilelog"
   RuntimeError: unexpected remotefilelog header: illegal format
 
 Verify detection and remediation when remotefilelog.validatecachelog is set