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

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

Fix:

    In test_stream_names line 43:
            echo "Created CTF trace \`$(cygpath -m ${temp_output_dir})\\trace\`." > "$temp_expected_stdout"
                 ^-- SC2028 (info): echo may not expand escape sequences. Use printf.
                                                   ^----------------^ SC2086 (info): Double quote to prevent globbing and word splitting.

    Did you mean:
            echo "Created CTF trace \`$(cygpath -m "${temp_output_dir}")\\trace\`." > "$temp_expected_stdout"

    In test_assume_single_trace line 41:
            echo "Created CTF trace \`$(cygpath -m ${trace_dir})\`." > "$temp_expected_stdout"
                                                   ^----------^ SC2086 (info): Double quote to prevent globbing and word splitting.

    Did you mean:
            echo "Created CTF trace \`$(cygpath -m "${trace_dir}")\`." > "$temp_expected_stdout"

Change-Id: I483ce2ba6c35c2ca191bf5ca1352ac97a0a9c157
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8637