tests/cli/test-intersection.sh: rewrite test to avoid shellcheck warning

tests/cli/test-intersection.sh: rewrite test to avoid shellcheck warning

Shellcheck 0.10.0 shows:

    $ shellcheck -x test-intersection.sh

    In test-intersection.sh line 36:
    	ok $? "$totalevents events in the whole trace"
               ^-- SC2319 (warning): This $? refers to a condition, not a command. Assign to a variable to avoid it being overwritten.

    For more information:
      https://www.shellcheck.net/wiki/SC2319 -- This $? refers to a condition, no...

I think that Shellcheck is confused with what we're trying to do and
that the code is actually correct. Nevertheless, change the code to
circumvent this by using `is`, which looks nicer anyway.

This is not seen on the CI, since it currently runs Shellcheck 0.9.0.

Change-Id: I7bb861b478046ab327c5950c9deefa52953bc854
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/13316
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>