Extract xlog.test into an own test suite. Deal with the collateral damage.

Extract xlog.test.py into an own test suite, in preparation
for a split into many tests.
xlog.test.py restarts the server all the time anyway, there is
no point to keep all tests in the same file.

Deal with collateral damange of this seemingly innocuous move:

 * fix an assertion failure in user.drop(), user_has_data().
   The check was using a wrong index.
 * fix test-run.py to leave cores around in case of a server crash
   (in a friendly gesture to develoeprs, and joining the ranks of
   such extremely succefful applications as apport and systemd, it used to
   delete them)
 * update bootstrap.snap to provide format for _index space
 * make a bunch of tests safe to the order in which they are executed
 * use space name rather than space id for error messages more often,
   since space id is very volatile after introduction of max_id sequence
   for space ids.