test: dirty fix for some flaky replication-luatest tests

The fixed tests often failed with

  fail | 2023-03-01 15:54:30.550 [3724975] main/103/server_instance.lua
  F> can't initialize storage: unlink, called on fd 63,
  aka unix/:(socket), peer of unix/:(socket): Address already in use

We fixed a similar issue in commit 3d3e9dea ("test: fix flaky
box-luatest/gh_7917_log_row_on_recovery_error_test") by using unique
instance names. Let's do the same here.

NO_DOC=testing stuff
NO_TEST=testing stuff
NO_CHANGELOG=testing stuff