tests: make test-shelve.t timing-independent

It was sometimes taking 2s for me (not the "1s" the test expected).

tests/test-shelve.t

@@ -1692,7 +1692,7 b' Unshelve respects --keep even if user in'
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ echo 3 >> file && hg ci -Am 13
   $ hg shelve --list
-  default         (1s ago)    changes to: 1
+  default         (*s ago)    changes to: 1 (glob)
   $ hg unshelve --keep
   unshelving change 'default'
   rebasing shelved changes