test.py: add a log file
test.py: add a log file

Going forward I'd like to make terminal output brief&tabular,
but some test details are necessary to preserve so that a failure
is easy to debug. This information now goes to the log file.

- open and truncate the log file on each harness start
- log options of each invoked test in the log, so that
  a failure is easy to reproduce
- log test result in the log

Since tests are run concurrently, having an exact
trace of concurrent execution also helps
debugging flaky tests.

avikivity/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla