[PLINK-405] regression tests

Added test to cover PLINK-405 - passUserPrincipalToAttributeManager attribute of Picketlink's IDPWebBrowserSSOValve.  It's a regression test for attribute mapping in Identity Provider (IdP) when SPNEGO (Kerberos) authentication is used.

The test itself is located in `org.jboss.as.test.integration.security.picketlink.SAML2AttributeMappingTestCase`.

The WildFly version of the test is marked with `@Ignore` because of the AS7-6796 issue (SPNEGO support in Undertow).