add srp X

test/srp_vectors.js

@@ -4,6 +4,7 @@ sjcl.test.vector.srp = [
   I: "alice",
   P: "password123",
   s: "BEB25379D1A8581EB5A727673A2441EE",
+  x: "94B7555AABE9127CC58CCF4993DB6CF84D16C124",
   v: "7E273DE8696FFC4F4E337D05B4B375BEB0DDE1569E8FA00A9886D8129BADA1F1822223CA1A605B530E379BA4729FDC59F105B4787E5186F5C671085A1447B52A48CF1970B4FB6F8400BBF4CEBFBB168152E08AB5EA53D15C1AFF87B2B9DA6E04E058AD51CC72BFC9033B564E26480D78E955A5E29E7AB245DB2BE315E2099AFB",
   known_group_size: 1024
 }