[ravenwood] Better log redirection to stdout

Now `export RAVENWOOD_LOG_OUT=-` will redirect ravenwood log to
atest's stdout, so that `atest ... | tee atest.log` now works as expected.

Flag: EXEMPT host test change only
Bug: 292141694
Test: RAVENWOOD_LOG_OUT=- atest RavenwoodMinimumTest`
Change-Id: I4fe1c4f13a2960fe78430f0a57102b04b1403670