tests: silence "variable/expression in single quote" shellcheck warnings

tests: silence "variable/expression in single quote" shellcheck warnings

shellcheck complains that we use a variable in single quotes:

    In test_trace_copy line 63:
                    uniq_ts_cnt="$("${BT_TESTS_AWK_BIN}" '{ print $1 }' < "${text_output1}" | sort | uniq | wc -l)"
                                                         ^------------^ SC2016: Expressions don't expand in single quotes, use double quotes for that.

In other cases, it's because of backticks (`) in a single quote string.

Silence these warnings locally by putting the appropriate comment:

    # shellcheck disable=SC2016

Reported-by: shellcheck
Change-Id: Ib2cfa73f84ba8746d3793e49721ead171c24dd99
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2418
Tested-by: jenkins <jenkins@lttng.org>