tests/lib/conds: store triggers in a vector

tests/lib/conds: store triggers in a vector

An upcoming patch needs to dynamically create and append triggers to the
trigger list.  Make the container a vector.  For simplicity, update
`condMain` to take a reference to a vector of unique pointers of
triggers (rather than inventing some abstraction).

Change-Id: Ic5e2fe6da151434ec515c041d4e1096193d29221
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12115
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>