Fix wchar.exp test case per review

A recent patch of mine modified wchar.exp, but I failed to notice one
part of the review.  This patch updates the code to conform to the
review comments.

gdb/testsuite/gdb.base/wchar.exp

diff --git a/gdb/testsuite/gdb.base/wchar.exp b/gdb/testsuite/gdb.base/wchar.exp
index 7f09e81..f0e4777 100644
--- a/gdb/testsuite/gdb.base/wchar.exp
+++ b/gdb/testsuite/gdb.base/wchar.exp
@@ -74,5 +74,5 @@
 gdb_test "printf \"%ls\\n\", 0" "\\(null\\)"
 
 # From PR exp/33124 - a bug when converting escapes.
-set result [string_to_regexp {L'\\'}]
-gdb_test "print L'\\\\'" " = $decimal $result"
+set wbs {L'\\'}
+gdb_test "print $wbs" " = $decimal [string_to_regexp $wbs]"