tests/lib: C++ify `conds/utils.cpp` a bit

tests/lib: C++ify `conds/utils.cpp` a bit

Change `utils.cpp` to fit a bit more our C++ style.

 - use an anonymous namespace
 - use camel-case for functions
 - use `bt2s::span` for the list of triggers
 - declare functions as `noexcept`

I didn't update the contents of `utils.hpp`, because I expect this to
change in some following patches, while I expect `utils.cpp` to stay
pretty much as-is.

Change-Id: I9dc01a22b3e9ceb2d90cd35c9f6ca93d1afe248f
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11794
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>