tests: remove unnecessary '^' from 'expr' regular expression

As Brandon noticed, a regular expression match given to 'expr' is already
anchored at the beginning.  Some versions of expr even complain about this.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t7005-editor.sh b/t/t7005-editor.sh
index 5257f4d..6355698 100755
--- a/t/t7005-editor.sh
+++ b/t/t7005-editor.sh
@@ -13,7 +13,7 @@
 
 '
 
-if ! expr "$vi" : '^[a-z]*$' >/dev/null
+if ! expr "$vi" : '[a-z]*$' >/dev/null
 then
 	vi=
 fi