test: write log file to current directory

test: write log file to current directory

instead of writing log files to '/tmp', write them to current directory.

by default, ctest runs the tests in ${CMAKE_CURRENT_BINARY_DIR}, so, in
this case, the log files are located in build/src/test/. this ensure
that current user has enough previledge to remove the log files.

this also allows multiple instances of 'unittest_log' to run in parallel.

See-also: http://tracker.ceph.com/issues/36737
Signed-off-by: Kefu Chai <kchai@redhat.com>