test-permissions: add a chmod +w to allow the directory to be removed

tests/test-permissions

@@ -13,5 +13,6 b' hg verify 2>/dev/null || echo verify fai'
 chmod -w .hg/store/data/a.i
 echo barber > a
 hg commit -m "2" -d "1000000 0" 2>/dev/null || echo commit failed
-chmod -w ../t
+chmod -w .
 hg diff --nodates
+chmod +w .