tests: use -z / -n to test for string empty / non-empty

shellcheck gives many warnings of this kind

    In env.sh.in line 44:
    if [ "x${BT_TESTS_CC_BIN:-}" = "x" ]; then
         ^---------------------^ SC2268 (style): Avoid x-prefix in comparisons as it no longer serves a purpose.

    Did you mean:
    if [ "${BT_TESTS_CC_BIN:-}" = "" ]; then

Fix them by using test's -z and -n switches, which test for empty and
non-empty strings, respectively.  In some cases, we are checking for the
string to have a specific value ( = "x1"), in which case I left the
comparison, but just removed the leading x's.

Change-Id: I3cd841d881916696419d540ed629e685d2edce9d
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8635
Tested-by: jenkins <jenkins@lttng.org>