support/testing: make crossbar to use umsgpack

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] https://github.com/crossbario/autobahn-python/commit/ea019b80427c9f6d43938631f8143bf7c993b4e7
[2] https://github.com/crossbario/autobahn-python/commit/fe70ceebe04849470b0c81b26ff6e65e709b0d33

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>