`server:grep_log()` doesn't find string logged in preceding `server:exec()`

[locker]

One might expect that server:grep_log() called immediately after server:exec() that writes something to the server log will surely find the logged string, however it isn't necessarily true: since logging goes through a pipe and then processed by a luatest fiber, one might need to yield before they can find the last logged message. Commit 2f16ac7, which reworked the luatest logging subsystem, increased the probability of failures like that. In particular, using luatest 1.1.0 with Tarantool makes a few tests using server:grep_log() fail with almost 100% probability, see tarantool/test-run#455 (comment). Instead of fixing those tests, we should make server:grep_log() more reliable.