test: fix flaky 'test_ignore_with_force_recovery'

From time to time we can see failures of one of `box-luatest` tests.
It's `gh_6794_recover_nonmatching_xlogs.test_ignore_with_force_recovery`
test:

    [021] box-luatest/gh_6794_recover_nonmatching_xlogs_>
    [021] not ok 2 box-luatest.gh_6794_recover_nonmatching_xlogs.test_ignore_with_force_recovery #
    [021] Rejected result file: /tmp/tnt/rejects/box-luatest/gh_6794_recover_nonmatching_xlogs.reject
    [021] [ fail ]

More detailed failure log:

    not ok 2	box-luatest.gh_6794_recover_nonmatching_xlogs.test_ignore_with_force_recovery
    #   .../tarantool/test/luatest_helpers/server.lua:99: Waiting for "readiness" on server master-UCg6PXluSqYt (PID 85920) timed out
    #   stack traceback:
    #   	.../tarantool/test/luatest_helpers/server.lua:99: in function 'wait_for_readiness'
    #   	.../tarantool/test/luatest_helpers/server.lua:146: in function 'start'
    #   	...t/box-luatest/gh_6794_recover_nonmatching_xlogs_test.lua:32: in function 'box-luatest.gh_6794_recover_nonmatching_xlogs.test_ignore_with_force_recovery'
    #   	...
    #   	[C]: in function 'xpcall'

If we take a look at the log of a tarantool server that is used for
testing, we will see the failure reason:

    2022-02-04 21:06:12.063 [85920] main/103/default.lua evio.c:240 E> SocketError: unlink, called on fd 27, aka unix/:(socket), peer of unix/:(socket): Address already in use
    2022-02-04 21:06:12.063 [85920] main/103/default.lua F> can't initialize storage: unlink, called on fd 27, aka unix/:(socket), peer of unix/:(socket): Address already in use

Both tests from the gh_6794_recover_nonmatching_xlogs_test.lua file used
the same alias (master) for the tarantool server, hence in both tests
the tarantool server used a socket with the same name (master.iproto).
So we had a race here: if the socket from the previous test was released
in time, our test succeeded, otherwise, failed.

To fix the issue it was decided to create tarantool servers with
different aliases. This gives us sockets with different names, hence no
socket intersection.

Closes tarantool/tarantool-qa#151

NO_CHANGELOG=test stuff
NO_DOC=testing stuff