test: refactor gh_6036_qsync_order test

The test involves creating a manual split-brain between nodes r1 and r2.
After the split-brain detection introduction it's impossible to reuse
the nodes in the next test without recreating them.

Let's fix that by switching nodes r1 and r3. Now there's a split-brain
between (r1, r2) and r3, and r3 isn't used in the following tests and
may be safely deleted.

Follow-up #5295

NO_DOC=refactoring
NO_CHANGELOG=refactoring

Signed-off-by: Serge Petrenko <sergepetrenko@tarantool.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>