tests: fix shellcheck warnings in plugins/src.ctf.fs/query/test_query_metadata_info

tests: fix shellcheck warnings in plugins/src.ctf.fs/query/test_query_metadata_info

Fix:

    In test_query_metadata_info line 32:
    this_dir_build="$BT_TESTS_BUILDDIR/$this_dir_relative"
    ^------------^ SC2034: this_dir_build appears unused. Verify use (or export if used externally).

    In test_query_metadata_info line 41:
	    local temp_stdout_output_file="$(mktemp -t actual_stdout.XXXXXX)"
		  ^---------------------^ SC2155: Declare and assign separately to avoid masking return values.

    In test_query_metadata_info line 42:
	    local temp_stderr_output_file="$(mktemp -t actual_stderr.XXXXXX)"
		  ^---------------------^ SC2155: Declare and assign separately to avoid masking return values.

Change-Id: I747d8092bf4625a748afb313b094e6782824c407
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reported-by: shellcheck
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3138
Tested-by: jenkins <jenkins@lttng.org>