Fix JMeter tests

Right now some JMeter tests fail or they simply work incorrectly (like the page is 500 error, but it still thinks it's a successful result).

We need to fix these tests:

• Those that fail should not fail - if there are wrong URLs, they should be fixed. If data do not pass validation, we need to fix that. All requests should be valid.
• If JCommune returns error page (which is actually 200 OK), we need to figure out why and fix the test scenario to send correct requests.

NB: performance tests may use pre-defined users to login to forum. If current tests refer to non-existent users, they should be replaced by users in sample-forum.sql.