Update merge test

This MR changes the open mode to read-only of the intermediate "spec h5" file during merge. This was probably the cause of random test failure in continuous integration (see #38 (closed)), but it is hard to be sure as it is random failure... It's anyway better to open this file as read-only. I managed to reproduce the failure once locally while upgrading h5py from 2.7.1 to 2.8.0, and didn't get any error with this patch... so far.

And, a bit of test clean up.

Related (hopefully closes, let's see) #38 (closed)