support/testing: make crossbar to use umsgpack

Since version 19.3.3, python-autobahn assumes that when CPython is used
msgpack will be used as well [1]. But it still allows the user to
override this behavior by setting an environment variable [2] to use
umsgpack.

Make the test to explicitly use umsgpack since it is part of the minimal
config (python-crossbar selects python-u-msgpack).

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/269139430

[1] ea019b8042
[2] fe70ceebe0

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

support/testing/tests/package/sample_python_crossbar.py

@@ -1,3 +1,5 @@
+import os
 import crossbar
 
+os.environ["AUTOBAHN_USE_UMSGPACK"] = "1"
 crossbar.run(["version"])