Use $decimal in timestamp.exp

This patch fixes a review comment by Tom de Vries.  He pointed out
that the new timestamp.exp should use the $decimal convenience regexp.

Author: tromey

gdb/testsuite/gdb.base/timestamp.exp

@@ -21,4 +21,4 @@ clean_restart
 gdb_test_no_output "set debug expression 1"
 gdb_test_no_output "set debug timestamp 1"
 
-gdb_test "print 23" "\[0-9\]+\\.\[0-9\]+ Operation: OP_LONG.* = 23"
+gdb_test "print 23" "$decimal\\.$decimal Operation: OP_LONG.* = 23"