Log stacktraces to file

Add an option to log the "BACKTRACE" message (here) to a file. The human-readable backtrace can be kept as-is in case the symbolized version is not available for any reason.

We'd like to use this functionality to automatically symbolize all backtraces run in Evergreen as part of the linked project. The symbolizer would run after a test fails to find any stacktraces in the files generated by the servers.

This can be a test-only functionality and does not have to be configurable. The location of the log files can be fixed (relative to the data dir) to simplify implementation.

If this can be enabled with --setParameter enableTestCommands=true, a separate command line option would not be needed