XSS test types changed, new `stored_xss` type introduced

[aborovsky]

XSS test types change is coming.

At the moment there 2 XSS-related test types supported:

1. xss - produce Stored XSS and Reflective XSS.
2. dom_xss - Produce Reflective XSS which gets renamed with MEtaDAta..

New supported XSS tests set will be:

1. xss - corresponds to Cross-Site Scripting test.
2. stored_xss - corresponds to new Stored Cross-Site Scripting test.
3. dom_xss - corresponds to Client-Side XSS test. This type is deprecated, will be decommissioned.