Add xtesting in upper-constraints.txt
Add xtesting in upper-constraints.txt

functest-kubernetes requires xtesting and still reads this file.

Change-Id: I18d357cc88ae9a7e80aca568d408823291dc2ab3
Signed-off-by: Cédric Ollivier <d48310251a4a484d041bc5d09a9ac4d86d20f793@orange.com>

mywulin/functest,opnfv/functest,opnfv/functest,mywulin/functest