tests/lib/conds/utils.cpp: condMain(): unset `BABELTRACE_EXEC_ON_ABORT`

tests/lib/conds/utils.cpp: condMain(): unset `BABELTRACE_EXEC_ON_ABORT`

This test willingly makes libbabeltrace2 abort, therefore
`BABELTRACE_EXEC_ON_ABORT` gets executed each time, and we don't want
that in the testing context.

condMain() is the closest to where the abort occurs: this means you may
run `tests/lib/conds/conds-triggers` directly and
`BABELTRACE_EXEC_ON_ABORT` won't impact the program.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I72f21cfcbe2ec8b1f48ec0703c42b3b1d710283c
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11818
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>