luatest: eliminate luatest_helpers/asserts.lua

The luatest_helpers/asserts.lua module confuses developers because
luatest itself has the asserts.lua module as well. It is unclear which
one should be used and when because it is hard to remember what stuff
provides each of them.

Actually, luatest_helpers/asserts.lua has functions that are not really
about asserting and can be safely moved to more proper places like the
luatest_helpers/server.lua and luatest_helpers/cluster.lua modules.

The `assert_server_follow_upstream` function logically belongs to server
stuff, so it was moved to server.lua and slightly changed (renamed into
`assert_follows_upstream` and `eval` calls were replaced by `exec`).

The `wait_fullmesh` function logically belongs to cluster stuff, so it
was moved to cluster.lua and slightly changed (now it accepts one param
that can include wait timeout and delay, otherwise, defaults applied).

Thus, the luatest_helpers/asserts.lua module was dropped and tests using
this module were updated.

Part of tarantool/luatest#192

NO_DOC=testing stuff
NO_CHANGELOG=testing stuff