conftest: Fix tests hang since elasticsearch 7.16 release
Since we are not using the elasticsearch startup script, we must explicitely set the LIBFFI_TMPDIR environment variable or elasticsearch will fail to start.
See https://www.elastic.co/guide/en/elasticsearch/reference/current/executable-jna-tmpdir.html
Closes #3803 (closed)
Migrated from D6828 (view on Phabricator)