test.py: split test output per test mode
test.py: split test output per test mode

Store test temporary files and logs in ${testdir}/${mode}.
Remove --jenkins and --xunit, and always write XML
files at a predefined location: ${testdir}/${mode}/xml/.

Use .xunit.xml extension for tests which XML output is
in xunit format, and junit.xml for an accumulated output
of all non-boost tests in junit format.

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