test-export: be more aggressive about quoting ^

An upcoming change to check-code will notice this isn't quoted
enough. Presumably it's been fine by luck all this time.

tests/test-export.t

@@ -159,7 +159,7 b' No filename should be printed if stdout '
 Checking if only alphanumeric characters are used in the file name (%m option):
 
   $ echo "line" >> foo
-  $ hg commit -m " !\"#$%&(,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_\`abcdefghijklmnopqrstuvwxyz{|}~"
+  $ hg commit -m " !\"#$%&(,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]"'^'"_\`abcdefghijklmnopqrstuvwxyz{|}~"
   $ hg export -v -o %m.patch tip
   exporting patch:
   ____________0123456789_______ABCDEFGHIJKLMNOPQRSTUVWXYZ______abcdefghijklmnopqrstuvwxyz____.patch