Right now it's logged to "R-unit-tests.log". Jenkins currently only archives files named "unit-tests.log", and this is what all other modules (e.g. SQL, network, REPL) use.
1. We should be consistent
2. I don't want to reconfigure Jenkins to accept a different file

shivaram

Author: andrewor14 <andrew@databricks.com>
Author: Andrew Or <andrew@databricks.com>

Closes #6807 from andrewor14/r-logs and squashes the following commits:

96005d2 [andrewor14] Nest unit-tests.log further until R
407c46c [andrewor14] Add target to log path
d7b68ae [Andrew Or] Log R unit test output to "unit-tests.log"