ensure that coverage is aggregated

Make sure that pytest-cov will aggregate the coverage resulsts from the
cram test run.

test.sh

@@ -7,7 +7,7 @@ echo "Testing command line interface with cram"
 COVERAGE=1 cram --verbose $@  test_cmdline/*.cram
 cram_exit=$?
 echo "Executing unit tests with pytest"
-PYTHONPATH=. pytest --cov=bloscpack test
+PYTHONPATH=. pytest --cov=bloscpack --cov-append test
 pytest_exit=$?
 if [ $cram_exit -gt 0 ] || [ $pytest_exit -gt 0 ] ; then
     echo "some tests failed"