implement new test "style": valgrind-logfile. This dumps everything to

implement new test "style": valgrind-logfile. This dumps everything to
log files named <test>-valgrind.log and only reports the LEAK SUMMARY to
stderr in case of reported errors. On travis, currently the logfiles
are effectively lost, but this all avoids problems with total test outputs
exceeding the current travis limit of 4MB...