UNDER --SKIP-GRANT-TABLES

 When running in --skip-grant-tables the effective user for the
 session was set to ''@'' by Security_context::skip_grants().

 When you run FLUSH PRIVILEGES in one such session it starts
 accepting normal ACL commands, but since the session stays as
 as initialized during the --skip-grant-tables mode.

 ''@'' collides with the restriction that proxy users
 (user name '') cannot be setting passwords.

 Fixed by setting the effective user to
 "skip-grants user"@"skip-grants host" instead of ''@''.

 Added a test case.
 Fixed existing tests that were relying on the old value.