test: fix server:grep_log() finding server restarts

grep_log() was ported from test_run.lua and relied on string "Starting
instance" being shown on instance restart. Luatest doesn't leave such
messages in instance logs, so better rely on tarantool version output to
detect instance restarts.

In-scope-of #5272

NO_DOC=testing
NO_TEST=testing
NO_CHANGELOG=testing