Convert Simpletest tests to BrowserTestBase

Patch queued to test, thanx a lot for conversion, I missed that tests were not converted yet

proper title

Here's a clean-up of useless changes, actually I'd prefer to split patches for tests conversion & clean-up

The session failure goes from \Drupal\Tests\masquerade\Functional\MasqueradeWebTestBase::assertSessionByUid() which starting session but it's not in testing browser

The hack breaks execution and tests now should pass

Rework of method to use string comparison instead of hacking decode session.

another round, somehow locally tests pass

The only thing I can imagine is that testbot using to run tests in subdir but that tells nothing why response code 500

Finally tests are fixed in #2951196: Permission Denied when trying to masquerade when site in subdir
It may have collision with #2930942: Convert module Masquerade to use short array syntax (new coding standard).

Getting in