Fix: consider index of all files for data stream groups with multiple files

Fix: consider index of all files for data stream groups with multiple files

The indexes associated to data stream file groups (dsfg) with multiple
files are incorrect.

The first time we process a file for a given dsfg, the group is created
using the index of that file.  Up to that point, it's fine.  When
processing the other files for that dsfg, the index is never used.  The
result is that the index for that dsfg only covers the time range of the
first file that was parsed, instead of the whole data stream.

Fix it by merging the index of the added file in
add_ds_file_to_ds_file_group, in the case where we add the file to an
existing group.

A test is added for this, in the form of a trace infos query.  The
implementation of this query uses the index data directly.

Change-Id: If1afd68be347210d1e258f8115b50547bc773760
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2212
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>