tests: add test demonstrating regression in path audit

D785 regressed behavior in path auditing: files can be deleted if they have a
path that conflicts with a filename from a malicious remote or bundle.

This test demonstrates the problem - the file should not have been deleted.

Differential Revision: https://phab.mercurial-scm.org/D1156

tests/test-audit-path.t

@@ -119,9 +119,14 b' attack ../test'
 
   $ hg manifest -r3
   ../test
+  $ mkdir ../test
+  $ echo data > ../test/file
   $ hg update -Cr3
   abort: path contains illegal component: ../test (glob)
   [255]
+  $ cat ../test/file
+  cat: ../test/file: No such file or directory
+  [1]
 
 attack /tmp/test