replication: fix flaky gh-3160-misc... test

The test fails quite often with one of the following results.

Either

[001] @@ -60,7 +60,7 @@
[001]  ...
[001]  test_timeout()
[001]  ---
[001] -- true
[001] +- false
[001]  ...
[001]  test_run:cmd("switch default")
[001]  ---
[001]

Or

[034]  test_timeout()
[034]  ---
[034] -- true
[034] +- error: 'replicas are not in the follow status'
[034]  ...

Both errors are caused by wait_cond checking saved
`box.info.replication` values instead of actual `box.info.replication`
output. Fix this.

Also, the test's quite long for no reason.

The wait_cond waiting for replication to break lasts a whole
`replication_timeout`, which is excess.

The test's idea, as stated in the commit that has introduced it
(195d4462: Send relay heartbeat if wal changes won't be send) is to
constantly relay some data to the remote peers and make
it so their relays never send heartbeats.

If we wait for a whole replication timeout between inserts, there's a high
chance of peer relays waking up naturally (after replication_timeout passes).

In this case the test tests nothing. Fix the issue by waiting for ~ 1/5
of replication_timeout in between the despatches. This also reduces test
run time from ~4.5 to ~1.5 seconds on my machine.

Remove the test from fragile list, since it shouldn't be flaky anymore.

Closes #4940