tests: Use DYLD_LIBRARY_PATH on macOSX

tests: Use DYLD_LIBRARY_PATH on macOSX

We could also use DYLD_FALLBACK_LIBRARY_PATH.

The dyld manual have this note regarding System Integrity Protection
(SIP).

  Note: If System Integrity Protection is enabled, these environment
  variables are ignored when executing binaries protected by System
  Integrity Protection.

If using restricted executables (e.g python), the DYLD_* will not be
honoured.

It might prove to be a problem in the wild. I'm not sure how to mitigate
this.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Change-Id: I9d62c4615f01e3a60d3068a13b5e2b1061aef6b4
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1653
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>