tests: Add ".exe" when running on a mingw system

tests: Add ".exe" when running on a mingw system

libtool wrapper fail to resolve executable without ".exe" suffix on
mingw system.

tests/ctf-writer/test_ctf_writer:

  babeltrace2:./.libs/lt-babeltrace2.c:237: FATAL: couldn't find ./../utils/../../src/cli/babeltrace2.
  not ok 325 - Babeltrace could read the resulting trace

Change-Id: I4e916077925b34745e6b70d14d86e263427d081e
Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1611
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>