fix(tests): Fix gh_7860_syslog_json_test broken due to memtx_sort_threads

test-run.py sets OMP_NUM_THREADS which causes this test to misbehave:
it doesn't expect to see a warning regarding the use of this env var
as the first log line.

Alternatively, we could update test-run's submodule, but in practice
this breaks other tests e.g. replication-luatest/linearizable_test.lua.

NO_DOC=internal
NO_TEST=internal
NO_CHANGELOG=internal