tests: HGRCPATH must be exported after assignment stable

tests: HGRCPATH must be exported after assignment

- at least on Solaris

--- a/tests/test-wireproto.t	Fri Jun 24 23:25:42 2011 +0300
+++ b/tests/test-wireproto.t	Sat Jun 25 01:55:13 2011 +0200
@@ -57,6 +57,7 @@
 HTTP without the httpheader capability:
 
   $ HGRCPATH="`pwd`/repo/.hgrc"
+  $ export HGRCPATH
   $ CAP=httpheader
   $ . "$TESTDIR/notcapable"