[Test] 411_logging: read per-suite rspamd output, not global .last

[Test] 411_logging: read per-suite rspamd output, not global .last

save_run_results writes each rspamd's logs to two destinations: the
stable per-suite/per-test directory under robot-save/, and a global
robot-save/<file>.last "convenience" copy of the most recent run.

The three 411_logging tests asserted on the .last copies. Under
pabot another worker can teardown -- and overwrite the .last files
-- between this suite's Rspamd Teardown saving them and the
assertion reading them, so the assertion ends up running against a
different suite's rspamd output and matching the wrong format.

Switch to the per-suite paths
(robot-save/${SUITE_NAME}/rspamd.stderr for the console suites,
robot-save/${SUITE_NAME}/${TEST_NAME}/rspamd.log for the JSON file
test). Those paths aren't shared across pabot workers.

Local: three back-to-back parallel runs of the 411_logging
directory pass 3/3 each time.