support/testing: simplify logging by keeping the log file open

support/testing: simplify logging by keeping the log file open

We currently call infra.smart_open() to open log files each time we
need to write to them.

Opening the file once in the constructor of Builder and Emulator and
writing to it whenever needed is simpler and slightly more efficient.

Remove smart_open and instead create a new open_log_file() function
which just opens the logfile. Also let it compute the filename, in
order to simplify even further the Builder and Emulator code.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>