tests: quote ${BT_CTF_TRACES_PATH} in test_trace_read and test_trace_copy

tests: quote ${BT_CTF_TRACES_PATH} in test_trace_read and test_trace_copy

Fixes these shellcheck errors:

    In test_trace_copy line 33:
    SUCCESS_TRACES=(${BT_CTF_TRACES_PATH}/succeed/*)
                    ^-------------------^ SC2206: Quote to prevent word splitting/globbing, or split robustly with mapfile or read -a.

    In test_trace_read line 29:
    SUCCESS_TRACES=(${BT_CTF_TRACES_PATH}/succeed/*)
                    ^-------------------^ SC2206: Quote to prevent word splitting/globbing, or split robustly with mapfile or read -a.

    In test_trace_read line 30:
    FAIL_TRACES=(${BT_CTF_TRACES_PATH}/fail/*)
                 ^-------------------^ SC2206: Quote to prevent word splitting/globbing, or split robustly with mapfile or read -a.

Reported-by: shellcheck
Change-Id: Ia13810be69d2bc57f6d6541887de0f9d75e3ffe5
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2417
Tested-by: jenkins <jenkins@lttng.org>