tests: fix shellcheck warnings in plugins/sink.ctf.fs/succeed/test_succeed

tests: fix shellcheck warnings in plugins/sink.ctf.fs/succeed/test_succeed

Fix:

    In test_succeed line 44:
	    local temp_out_trace_dir="$(mktemp -d)"
		  ^----------------^ SC2155: Declare and assign separately to avoid masking return values.

    In test_succeed line 73:
	    local temp_gen_trace_dir="$(mktemp -d)"
		  ^----------------^ SC2155: Declare and assign separately to avoid masking return values.

Change-Id: I1d7d419d8a11458200d6bf97b1558c8183315dce
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reported-by: shellcheck
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3141
Tested-by: jenkins <jenkins@lttng.org>